I'm encountering the same problems with my Dell Inspiron 1525 and Xubuntu 14.04 (Trusty Tahr) x86_64.  Most any display change (blanking, add/removing external monitor) results in a non-fatal error:

May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784402] CPU: 1 PID: 1546 Comm: Xorg Tainted: PF       W  O 3.13.0-24-generic #46-Ubuntu
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784407] Hardware name: Dell Inc. Inspiron 1525                   /0U990C, BIOS A11 03/10/2008
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784412]  0000000000000009 ffff88007918dbb8 ffffffff81715a64 0000000000000000
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784423]  ffff88007918dbf0 ffffffff810676bd ffff880035d0b000 0000000000000000
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784433]  ffff880035c17800 ffff880035c17af8 ffff880056704ca0 ffff88007918dc00
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784443] Call Trace:
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784456]  [<ffffffff81715a64>] dump_stack+0x45/0x56
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784467]  [<ffffffff810676bd>] warn_slowpath_common+0x7d/0xa0
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784475]  [<ffffffff8106779a>] warn_slowpath_null+0x1a/0x20
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784521]  [<ffffffffa00ba232>] intel_modeset_check_state+0x6f2/0x780 [i915]
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784568]  [<ffffffffa00ba355>] intel_set_mode+0x25/0x30 [i915]
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784615]  [<ffffffffa00babfb>] intel_crtc_set_config+0x7ab/0x9a0 [i915]
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784657]  [<ffffffffa000fe9d>] drm_mode_set_config_internal+0x5d/0xe0 [drm]
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784696]  [<ffffffffa0012d87>] drm_mode_setcrtc+0xf7/0x5e0 [drm]
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784725]  [<ffffffffa0003c22>] drm_ioctl+0x502/0x630 [drm]
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784743]  [<ffffffff811cc6e0>] do_vfs_ioctl+0x2e0/0x4c0
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784752]  [<ffffffff8109dd84>] ? vtime_account_user+0x54/0x60
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784761]  [<ffffffff811cc941>] SyS_ioctl+0x81/0xa0
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784771]  [<ffffffff8172663f>] tracesys+0xe1/0xe6
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.784776] ---[ end trace 7832e5d6c3fcdce6 ]---
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.790057] ------------[ cut here ]------------
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.790120] WARNING: CPU: 1 PID: 1546 at /build/buildd/linux-3.13.0/drivers/gpu/drm/i915/intel_display.c:9226 intel_modeset_check_state+0x62d/0x780 [i915]()
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.790123] encoder's hw state doesn't match sw tracking (expected 0, found 1)
May 20 19:03:18 jhansonxi-l4 kernel: [ 8205.790125] Modules linked in:...

Suspend/hibernate fails also.  Closing the lid results in a freeze.

The "encoder's hw state doesn't match sw tracking" seems to be the problem.  A fix for the same error was made last year:
http://linux-kernel.2935.n7.nabble.com/Re-Re-drm-i915-new-warning-regression-in-3-7-10-and-3-8-3-td631548.html
https://bugzilla.redhat.com/show_bug.cgi?id=962386

The current problem may be another regression.  Others encountered a similar problem with 3.13:
https://lkml.org/lkml/2013/12/17/796