Same happens regularly - dmesg contains loads of:

[10046.802408] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x0000060d
[10046.802417] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802420] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000176
[10046.802429] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802432] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x000003c9
[10046.802441] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802445] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000176
[10046.802453] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802457] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x000003ca
[10046.802465] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802469] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000186
[10046.802515] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802519] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x0000041c
[10046.802531] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802535] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000176
[10046.802544] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802547] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x0000041d
[10046.802556] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802559] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000186
[10046.802568] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802571] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x0000046f
[10046.802580] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802584] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000176
[10046.802593] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802596] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x00000470
[10046.802605] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802608] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000186
[10046.802617] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802621] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x000004c2
[10046.802630] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802633] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000176
[10046.802642] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802645] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x000004c3
[10046.802654] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802657] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000186
[10046.802667] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802670] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x00000515
[10046.802679] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802682] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000176
[10046.802691] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802694] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x00000516
[10046.802703] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802706] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000186
[10046.802715] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802719] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x00000568
[10046.802728] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802731] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000176
[10046.802740] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802743] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x00000569
[10046.802752] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802755] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000186
[10046.802764] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802767] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x000005bb
[10046.802777] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802780] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000176
[10046.802789] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802792] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x000005bc
[10046.802801] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802804] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000186
[10046.802813] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802817] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x00000000
[10046.802826] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802829] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000186
[10046.802838] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802841] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x0000060d
[10046.802850] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802853] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000187
[10046.802863] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802866] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x000003c9
[10046.802875] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802878] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000187
[10046.802887] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802890] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x000003ca
[10046.802899] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802902] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000197
[10046.802912] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802915] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x0000041c
[10046.802924] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802927] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000187
[10046.802936] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802939] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x0000041d
[10046.802948] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802951] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000197
[10046.802961] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802964] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x0000046f
[10046.802973] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802976] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000187
[10046.802985] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.802988] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x00000470
[10046.802997] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803000] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000197
[10046.803009] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803013] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x000004c2
[10046.803022] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803025] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000187
[10046.803034] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803037] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x000004c3
[10046.803046] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803049] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000197
[10046.803058] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803062] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x00000515
[10046.803071] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803074] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000187
[10046.803083] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803086] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x00000516
[10046.803095] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803098] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000197
[10046.803107] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803110] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x00000568
[10046.803119] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803123] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000187
[10046.803132] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803135] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x00000569
[10046.803144] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803147] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000197
[10046.803156] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803159] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x000005bb
[10046.803168] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803172] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000187
[10046.803181] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803184] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x000005bc
[10046.803193] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803196] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000197
[10046.803205] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803208] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x00000000
[10046.803217] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803220] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000197
[10046.803230] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803233] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x0000060d
[10046.803242] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803245] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x00000198
[10046.803254] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803257] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x000003c9
[10046.803266] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803269] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000198
[10046.803279] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803282] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x000003ca
[10046.803291] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803294] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x000001a8
[10046.803303] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803306] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x0000041c
[10046.803316] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803319] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000198
[10046.803328] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803331] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x0000041d
[10046.803340] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803343] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x000001a8
[10046.803352] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803355] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x0000046f
[10046.803365] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803368] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000198
[10046.803377] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803380] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x00000470
[10046.803389] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803392] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x000001a8
[10046.803401] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803404] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0600 data 0x000004c2
[10046.803413] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803416] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0604 data 0x00000198
[10046.803425] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803428] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x0608 data 0x000004c3
[10046.803437] nouveau E[  PGRAPH][0000:01:00.0]  ILLEGAL_MTHD ILLEGAL_CLASS
[10046.803440] nouveau E[  PGRAPH][0000:01:00.0] ch 3 [0x001fb24000 Xorg[19658]] subc 2 class 0x0000 mthd 0x060c data 0x000001a8
[10046.806480] nouveau E[     PFB][0000:01:00.0] trapped write at 0x0000000000 on channel 0x0001fb24 [Xorg[19658]] PGRAPH/DISPATCH/GRCTX reason: DMAOBJ_LIMIT
[10047.131817] nouveau E[     PFB][0000:01:00.0] trapped read at 0x0040990000 on channel 0x0001fb24 [Xorg[19658]] PGRAPH/TEXTURE/00 reason: PAGE_NOT_PRESENT

Not sure if I can provide any more useful information since it seems to happen randomly and independently of current keyboard/mouse interaction. The screen just froze without me sitting in front of the box.