This recently started happening to me also.  My recent changes-
- I have a samsung 4k monitor attached via displayport but I just started using my laptop screen at the same time (2 displays - was only using the 4k monitor OR the laptop display previously).
- I installed a Dell-provided pulseaudio patch to enable sound to go through the displayport connection so I can plug my headphones into the audio out jack on the monitor.

I wasn't doing anything exciting at the last time it happened - was running Ansible, had Chrome and Hipchat open along with a few terminals.

Here is the trace from dmesg:
Dec 29 18:04:13 tso-lpjg kernel: [13172.056836] ------------[ cut here ]------------
Dec 29 18:04:13 tso-lpjg kernel: [13172.056943] WARNING: CPU: 1 PID: 1913 at /build/buildd/linux-3.16.0/drivers/gpu/drm/i915/intel_display.c:3324 intel_crtc_
wait_for_pending_flips+0x16c/0x180 [i915]()
Dec 29 18:04:13 tso-lpjg kernel: [13172.056948] Modules linked in: xt_addrtype xt_conntrack aufs ip6table_filter ip6_tables vmnet(OE) vmw_vsock_vmci_transpor
t vsock vmw_vmci vmmon(OE) nfnetlink_queue nfnetlink_log nfnetlink 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 dm_crypt dell_wmi sparse_keymap intel_rapl x86_pkg_temp_thermal dell_
laptop intel_powerclamp coretemp arc4 dcdbas kvm_intel kvm crct10dif_pclmul crc32_pclmul ghash_clmulni_intel iwldvm mac80211 aesni_intel aes_x86_64 lrw gf128
mul glue_helper ablk_helper iwlwifi cryptd snd_hda_codec_realtek serio_raw snd_hda_codec_generic snd_hda_codec_hdmi cfg80211 uvcvideo videobuf2_vmalloc video
buf2_memops lpc_ich videobuf2_core snd_soc_rt5640 btusb v4l2_common videodev joydev media snd_hda_intel snd_soc_rl6231 snd_hda_controller snd_soc_core snd_hd
a_codec snd_compress snd_hwdep snd_pcm_dmaengine mei_me mei snd_pcm bnep rfcomm snd_seq_midi snd_seq_midi_event bluetooth snd_rawmidi shpchp 6lowpan_iphc snd
_seq snd_seq_device snd_timer dell_smo8800 snd i2c_hid dw_dmac soundcore dw_dmac_core snd_soc_sst_acpi i2c_designware_platform 8250_dw i2c_designware_core sp
i_pxa2xx_platform mac_hid parport_pc nfsd auth_rpcgss ppdev nfs_acl lp parport nfs lockd sunrpc fscache binfmt_misc btrfs xor raid6_pq hid_microsoft hid_gene
ric usbhid hid radeon i915 ttm i2c_algo_bit drm_kms_helper e1000e drm psmouse ahci libahci ptp sdhci_pci pps_core wmi sdhci_acpi video sdhci [last unloaded: 
vmnet]
Dec 29 18:04:13 tso-lpjg kernel: [13172.057109] CPU: 1 PID: 1913 Comm: Xorg Tainted: G           OE 3.16.0-28-generic #38-Ubuntu
Dec 29 18:04:13 tso-lpjg kernel: [13172.057113] Hardware name: Dell Inc. Latitude E6540/0725FP, BIOS A12 11/14/2014
Dec 29 18:04:13 tso-lpjg kernel: [13172.057116]  0000000000000009 ffff8804139b3c18 ffffffff81781eaa 0000000000000000
Dec 29 18:04:13 tso-lpjg kernel: [13172.057123]  ffff8804139b3c50 ffffffff8106fedd 0000000000000000 ffff880417c39000
Dec 29 18:04:13 tso-lpjg kernel: [13172.057128]  ffff880411e88210 ffff8804135cb000 ffff8804135cb000 ffff8804139b3c60
Dec 29 18:04:13 tso-lpjg kernel: [13172.057134] Call Trace:
Dec 29 18:04:13 tso-lpjg kernel: [13172.057152]  [<ffffffff81781eaa>] dump_stack+0x45/0x56
Dec 29 18:04:13 tso-lpjg kernel: [13172.057162]  [<ffffffff8106fedd>] warn_slowpath_common+0x7d/0xa0
Dec 29 18:04:13 tso-lpjg kernel: [13172.057169]  [<ffffffff8106ffba>] warn_slowpath_null+0x1a/0x20
Dec 29 18:04:13 tso-lpjg kernel: [13172.057219]  [<ffffffffc032604c>] intel_crtc_wait_for_pending_flips+0x16c/0x180 [i915]
Dec 29 18:04:13 tso-lpjg kernel: [13172.057231]  [<ffffffff810b9710>] ? prepare_to_wait_event+0x100/0x100
Dec 29 18:04:13 tso-lpjg kernel: [13172.057279]  [<ffffffffc0328b13>] intel_crtc_disable_planes+0x33/0x1c0 [i915]
Dec 29 18:04:13 tso-lpjg kernel: [13172.057286]  [<ffffffff8178703b>] ? __ww_mutex_lock+0x1b/0xb0
Dec 29 18:04:13 tso-lpjg kernel: [13172.057332]  [<ffffffffc0329a67>] haswell_crtc_disable+0x57/0x2e0 [i915]
Dec 29 18:04:13 tso-lpjg kernel: [13172.057339]  [<ffffffff81787c92>] ? mutex_lock+0x12/0x30
Dec 29 18:04:13 tso-lpjg kernel: [13172.057385]  [<ffffffffc032a4a7>] intel_crtc_update_dpms+0x67/0xa0 [i915]
Dec 29 18:04:13 tso-lpjg kernel: [13172.057432]  [<ffffffffc032eb19>] intel_connector_dpms+0x59/0x70 [i915]
Dec 29 18:04:13 tso-lpjg kernel: [13172.057477]  [<ffffffffc01a5e19>] drm_mode_obj_set_property_ioctl+0x399/0x3a0 [drm]
Dec 29 18:04:13 tso-lpjg kernel: [13172.057510]  [<ffffffffc01a5e50>] drm_mode_connector_property_set_ioctl+0x30/0x40 [drm]
Dec 29 18:04:13 tso-lpjg kernel: [13172.057534]  [<ffffffffc0194a4f>] drm_ioctl+0x1df/0x680 [drm]
Dec 29 18:04:13 tso-lpjg kernel: [13172.057548]  [<ffffffff811f5138>] do_vfs_ioctl+0x2c8/0x4a0
Dec 29 18:04:13 tso-lpjg kernel: [13172.057554]  [<ffffffff81784b2d>] ? __schedule+0x39d/0x890
Dec 29 18:04:13 tso-lpjg kernel: [13172.057560]  [<ffffffff811f5391>] SyS_ioctl+0x81/0xa0
Dec 29 18:04:13 tso-lpjg kernel: [13172.057567]  [<ffffffff81789eed>] system_call_fastpath+0x1a/0x1f
Dec 29 18:04:13 tso-lpjg kernel: [13172.057571] ---[ end trace 72247ec4c41b126d ]---