Appears to be Kernel bug in graphics driver:

[196179.037371] ------------[ cut here ]------------
[196179.037408] WARNING: CPU: 2 PID: 1548 at /build/buildd/linux-3.16.0/drivers/gpu/drm/i915/intel_display.c:3324 intel_crtc_wait_for_pending_flips+0x16c/0x180 [i915]()
[196179.037410] Modules linked in: usblp pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) xt_CHECKSUM iptable_mangle ipt_MASQUERADE iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack xt_tcpudp bridge stp llc iptable_filter ip_tables x_tables snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm snd_hda_intel joydev snd_hda_controller uvcvideo crct10dif_pclmul crc32_pclmul snd_hda_codec bnep rfcomm videobuf2_vmalloc snd_usb_audio videobuf2_memops ghash_clmulni_intel bluetooth videobuf2_core snd_usbmidi_lib snd_hwdep 6lowpan_iphc v4l2_common videodev media aesni_intel snd_pcm aes_x86_64 lrw gf128mul glue_helper i915 ablk_helper cryptd drm_kms_helper drm lpc_ich i2c_algo_bit mei_me snd_seq_midi
[196179.037448]  mei snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer shpchp snd soundcore video mac_hid binfmt_misc parport_pc ppdev lp parport btrfs xor raid6_pq hid_generic usbhid hid ahci libahci r8169 mii
[196179.037468] CPU: 2 PID: 1548 Comm: Xorg Tainted: G           OE 3.16.0-23-generic #31-Ubuntu
[196179.037470] Hardware name: Shuttle Inc. SH67H/FH67H, BIOS 2.02 05/07/2012
[196179.037472]  0000000000000009 ffff8803f0697bf8 ffffffff8177fcbc 0000000000000000
[196179.037475]  ffff8803f0697c30 ffffffff8106fd8d 0000000000000000 ffff8800363f8000
[196179.037478]  ffff8800b9808210 ffff8804075ae000 ffff8804075ae000 ffff8803f0697c40
[196179.037480] Call Trace:
[196179.037488]  [<ffffffff8177fcbc>] dump_stack+0x45/0x56
[196179.037493]  [<ffffffff8106fd8d>] warn_slowpath_common+0x7d/0xa0
[196179.037496]  [<ffffffff8106fe6a>] warn_slowpath_null+0x1a/0x20
[196179.037514]  [<ffffffffc0456ffc>] intel_crtc_wait_for_pending_flips+0x16c/0x180 [i915]
[196179.037519]  [<ffffffff810b9590>] ? prepare_to_wait_event+0x100/0x100
[196179.037536]  [<ffffffffc0459ac3>] intel_crtc_disable_planes+0x33/0x1c0 [i915]
[196179.037551]  [<ffffffffc045a090>] ironlake_crtc_disable+0x50/0x980 [i915]
[196179.037568]  [<ffffffffc039df53>] ? drm_modeset_lock+0x33/0xf0 [drm]
[196179.037571]  [<ffffffff81785ab2>] ? mutex_lock+0x12/0x30
[196179.037587]  [<ffffffffc045b457>] intel_crtc_update_dpms+0x67/0xa0 [i915]
[196179.037603]  [<ffffffffc045fac9>] intel_connector_dpms+0x59/0x70 [i915]
[196179.037617]  [<ffffffffc0394e19>] drm_mode_obj_set_property_ioctl+0x399/0x3a0 [drm]
[196179.037629]  [<ffffffffc0394e50>] drm_mode_connector_property_set_ioctl+0x30/0x40 [drm]
[196179.037638]  [<ffffffffc0383a4f>] drm_ioctl+0x1df/0x680 [drm]
[196179.037643]  [<ffffffff8121f9bc>] ? fsnotify+0x27c/0x350
[196179.037646]  [<ffffffff8121f9bc>] ? fsnotify+0x27c/0x350
[196179.037650]  [<ffffffff811f4bc8>] do_vfs_ioctl+0x2c8/0x4a0
[196179.037653]  [<ffffffff811e31a1>] ? __sb_end_write+0x31/0x60
[196179.037657]  [<ffffffff811e0d12>] ? vfs_write+0x1b2/0x1f0
[196179.037660]  [<ffffffff811f4e21>] SyS_ioctl+0x81/0xa0
[196179.037663]  [<ffffffff81787ced>] system_call_fastpath+0x1a/0x1f
[196179.037665] ---[ end trace a8a7a33a6d58937d ]---
[196186.075365] systemd-logind[1027]: New session c3 of user john.
[198711.366918] usb 3-2: USB disconnect, device number 8
[208844.807407] ------------[ cut here ]------------
[208844.807444] WARNING: CPU: 1 PID: 1548 at /build/buildd/linux-3.16.0/drivers/gpu/drm/i915/intel_display.c:3324 intel_crtc_wait_for_pending_flips+0x16c/0x180 [i915]()
[208844.807446] Modules linked in: usblp pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) xt_CHECKSUM iptable_mangle ipt_MASQUERADE iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack xt_tcpudp bridge stp llc iptable_filter ip_tables x_tables snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic intel_rapl x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel kvm snd_hda_intel joydev snd_hda_controller uvcvideo crct10dif_pclmul crc32_pclmul snd_hda_codec bnep rfcomm videobuf2_vmalloc snd_usb_audio videobuf2_memops ghash_clmulni_intel bluetooth videobuf2_core snd_usbmidi_lib snd_hwdep 6lowpan_iphc v4l2_common videodev media aesni_intel snd_pcm aes_x86_64 lrw gf128mul glue_helper i915 ablk_helper cryptd drm_kms_helper drm lpc_ich i2c_algo_bit mei_me snd_seq_midi
[208844.807484]  mei snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer shpchp snd soundcore video mac_hid binfmt_misc parport_pc ppdev lp parport btrfs xor raid6_pq hid_generic usbhid hid ahci libahci r8169 mii
[208844.807504] CPU: 1 PID: 1548 Comm: Xorg Tainted: G        W  OE 3.16.0-23-generic #31-Ubuntu
[208844.807506] Hardware name: Shuttle Inc. SH67H/FH67H, BIOS 2.02 05/07/2012
[208844.807508]  0000000000000009 ffff8803f0697bf8 ffffffff8177fcbc 0000000000000000
[208844.807511]  ffff8803f0697c30 ffffffff8106fd8d 0000000000000000 ffff8800363f8000
[208844.807514]  ffff8800b9808210 ffff8804075ae000 ffff8804075ae000 ffff8803f0697c40
[208844.807517] Call Trace:
[208844.807525]  [<ffffffff8177fcbc>] dump_stack+0x45/0x56
[208844.807530]  [<ffffffff8106fd8d>] warn_slowpath_common+0x7d/0xa0
[208844.807533]  [<ffffffff8106fe6a>] warn_slowpath_null+0x1a/0x20
[208844.807551]  [<ffffffffc0456ffc>] intel_crtc_wait_for_pending_flips+0x16c/0x180 [i915]
[208844.807556]  [<ffffffff810b9590>] ? prepare_to_wait_event+0x100/0x100
[208844.807572]  [<ffffffffc0459ac3>] intel_crtc_disable_planes+0x33/0x1c0 [i915]
[208844.807588]  [<ffffffffc045a090>] ironlake_crtc_disable+0x50/0x980 [i915]
[208844.807604]  [<ffffffffc039df53>] ? drm_modeset_lock+0x33/0xf0 [drm]
[208844.807607]  [<ffffffff81785ab2>] ? mutex_lock+0x12/0x30
[208844.807624]  [<ffffffffc045b457>] intel_crtc_update_dpms+0x67/0xa0 [i915]
[208844.807640]  [<ffffffffc045fac9>] intel_connector_dpms+0x59/0x70 [i915]
[208844.807653]  [<ffffffffc0394e19>] drm_mode_obj_set_property_ioctl+0x399/0x3a0 [drm]
[208844.807666]  [<ffffffffc0394e50>] drm_mode_connector_property_set_ioctl+0x30/0x40 [drm]
[208844.807675]  [<ffffffffc0383a4f>] drm_ioctl+0x1df/0x680 [drm]
[208844.807680]  [<ffffffff8121f9bc>] ? fsnotify+0x27c/0x350
[208844.807683]  [<ffffffff8121f9bc>] ? fsnotify+0x27c/0x350
[208844.807687]  [<ffffffff811f4bc8>] do_vfs_ioctl+0x2c8/0x4a0
[208844.807690]  [<ffffffff811e31a1>] ? __sb_end_write+0x31/0x60
[208844.807694]  [<ffffffff811e0d12>] ? vfs_write+0x1b2/0x1f0
[208844.807697]  [<ffffffff811f4e21>] SyS_ioctl+0x81/0xa0
[208844.807700]  [<ffffffff81787ced>] system_call_fastpath+0x1a/0x1f
[208844.807702] ---[ end trace a8a7a33a6d58937e ]---
[286854.767611] chromium-browse[2382]: segfault at 1f8 ip 00007f4f6ec4f63f sp 00007fffa543caf0 error 4 in i965_dri.so[7f4f6e8fc000+51c000]
[286858.424485] chromium-browse[2519]: segfault at 1f8 ip 00007fbdcfa3263f sp 00007fffead422a0 error 4 in i965_dri.so[7fbdcf6df000+51c000]
[286858.975365] chromium-browse[2556]: segfault at 1f8 ip 00007f274ed7563f sp 00007fff4b03a010 error 4 in i965_dri.so[7f274ea22000+51c000]
[287883.918632] chromium-browse[8266]: segfault at 1f8 ip 00007f942bb1e63f sp 00007fff5997fb10 error 4 in i965_dri.so[7f942b7cb000+51c000]
[287884.505711] chromium-browse[8408]: segfault at 1f8 ip 00007ff92a40163f sp 00007fffaa8e7e80 error 4 in i965_dri.so[7ff92a0ae000+51c000]
[287884.977205] chromium-browse[8452]: segfault at 1f8 ip 00007f024152f63f sp 00007fff74e351a0 error 4 in i965_dri.so[7f02411dc000+51c000]