Oh well spoke too soon:

Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980058] ------------[ cut here ]------------
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980068] WARNING: at /build/buildd/linux-2.6.31/lib/kref.c:43 kref_get+0x2d/0x30()
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980070] Hardware name: OptiPlex 745                 
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980072] Modules linked in: binfmt_misc vboxnetflt vboxnetadp vboxdrv kvm_intel kvm snd_hda_codec_analog snd_hda_intel snd_hda_codec snd_hwdep snd_pcm_oss snd_mixer_oss snd_pcm snd_seq_dummy snd_seq_oss ppdev iptable_filter snd_seq_midi ip_tables x_tables gspca_zc3xx gspca_main videodev v4l1_compat v4l2_compat_ioctl32 pl2303 usbserial snd_rawmidi snd_seq_midi_event psmouse serio_raw parport_pc dell_wmi dcdbas lp parport snd_seq snd_timer snd_seq_device snd soundcore snd_page_alloc usbhid dm_raid45 xor fbcon tileblit font bitblit softcursor radeon ttm drm i2c_algo_bit floppy tg3 intel_agp
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980114] Pid: 10, comm: events/1 Not tainted 2.6.31-18-generic #55-Ubuntu
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980116] Call Trace:
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980121]  [<ffffffff8105e828>] warn_slowpath_common+0x78/0xb0
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980125]  [<ffffffff8105e86f>] warn_slowpath_null+0xf/0x20
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980127]  [<ffffffff81279bed>] kref_get+0x2d/0x30
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980135]  [<ffffffffa007fdf4>] ttm_bo_delayed_delete+0x94/0x180 [ttm]
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980141]  [<ffffffffa007fee0>] ? ttm_bo_delayed_workqueue+0x0/0x40 [ttm]
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980146]  [<ffffffffa007fefa>] ttm_bo_delayed_workqueue+0x1a/0x40 [ttm]
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980150]  [<ffffffff81073865>] run_workqueue+0x95/0x170
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980153]  [<ffffffff810739e4>] worker_thread+0xa4/0x120
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980156]  [<ffffffff81078b90>] ? autoremove_wake_function+0x0/0x40
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980159]  [<ffffffff81073940>] ? worker_thread+0x0/0x120
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980162]  [<ffffffff810787a6>] kthread+0xa6/0xb0
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980165]  [<ffffffff8101312a>] child_rip+0xa/0x20
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980168]  [<ffffffff81078700>] ? kthread+0x0/0xb0
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980170]  [<ffffffff81013120>] ? child_rip+0x0/0x20
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980172] ---[ end trace c31a867b7006f62a ]---
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980238] CPU 0 
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980240] Modules linked in: binfmt_misc vboxnetflt vboxnetadp vboxdrv kvm_intel kvm snd_hda_codec_analog snd_hda_intel snd_hda_codec snd_hwdep snd_pcm_oss snd_mixer_oss snd_pcm snd_seq_dummy snd_seq_oss ppdev iptable_filter snd_seq_midi ip_tables x_tables gspca_zc3xx gspca_main videodev v4l1_compat v4l2_compat_ioctl32 pl2303 usbserial snd_rawmidi snd_seq_midi_event psmouse serio_raw parport_pc dell_wmi dcdbas lp parport snd_seq snd_timer snd_seq_device snd soundcore snd_page_alloc usbhid dm_raid45 xor fbcon tileblit font bitblit softcursor radeon ttm drm i2c_algo_bit floppy tg3 intel_agp
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980275] Pid: 9, comm: events/0 Tainted: G        W  2.6.31-18-generic #55-Ubuntu OptiPlex 745                 
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980277] RIP: 0010:[<ffffffff81114b3c>]  [<ffffffff81114b3c>] kfree+0x12c/0x140
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980283] RSP: 0018:ffff880137a91d10  EFLAGS: 00010246
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980285] RAX: 0100000000000000 RBX: ffffffffa00b35d3 RCX: ffff8800aed00128
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980287] RDX: 0000000000576cb8 RSI: ffffea000263f908 RDI: ffff8800aed97e00
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980289] RBP: ffff880137a91d30 R08: 0000000000000000 R09: 0000000000000000
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980291] R10: 0000000000000594 R11: 0000000000000000 R12: ffff8800aed97e00
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980293] R13: ffff8800aed97e3c R14: ffff8800aed97e3c R15: ffff880134c3f7a8
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980295] FS:  0000000000000000(0000) GS:ffff880028022000(0000) knlGS:0000000000000000
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980297] CS:  0010 DS: 0018 ES: 0018 CR0: 000000008005003b
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980299] CR2: 00007f7b37a18000 CR3: 000000011fd3f000 CR4: 00000000000026f0
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980301] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980303] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980306] Process events/0 (pid: 9, threadinfo ffff880137a90000, task ffff880137a85ac0)
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980309]  ffff880134c3f4a0 ffff8800aed97e00 ffff880134c3f330 ffff8800aed97e3c
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980312] <0> ffff880137a91d50 ffffffffa00b35d3 ffff880137a91da0 ffff8800aed97e00
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980316] <0> ffff880137a91d80 ffffffffa007ec6a ffff88011ec2c410 ffff8800aed97e3c
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980339]  [<ffffffffa00b35d3>] radeon_ttm_object_object_destroy+0x43/0x50 [radeon]
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980346]  [<ffffffffa007ec6a>] ttm_bo_release_list+0x8a/0xe0 [ttm]
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980351]  [<ffffffffa007ebe0>] ? ttm_bo_release_list+0x0/0xe0 [ttm]
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980354]  [<ffffffff81279b87>] kref_put+0x37/0x70
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980359]  [<ffffffffa007fe27>] ttm_bo_delayed_delete+0xc7/0x180 [ttm]
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980364]  [<ffffffffa007fee0>] ? ttm_bo_delayed_workqueue+0x0/0x40 [ttm]
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980369]  [<ffffffffa007fefa>] ttm_bo_delayed_workqueue+0x1a/0x40 [ttm]
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980373]  [<ffffffff81073865>] run_workqueue+0x95/0x170
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980375]  [<ffffffff810739e4>] worker_thread+0xa4/0x120
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980378]  [<ffffffff81078b90>] ? autoremove_wake_function+0x0/0x40
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980381]  [<ffffffff81073940>] ? worker_thread+0x0/0x120
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980384]  [<ffffffff810787a6>] kthread+0xa6/0xb0
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980387]  [<ffffffff8101312a>] child_rip+0xa/0x20
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980390]  [<ffffffff81078700>] ? kthread+0x0/0xb0
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980392]  [<ffffffff81013120>] ? child_rip+0x0/0x20
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980428]  RSP <ffff880137a91d10>
Jan 27 08:18:07 nsh-ubuntu kernel: [  263.980430] ---[ end trace c31a867b7006f62b ]---

Error description has been appended to similar error reported on a newer kernel:
https://bugs.freedesktop.org/show_bug.cgi?id=25079