Video locked up completely. Mouse wouldn't move, CTL-ALT-F1 wouldn't switch to emergency backup console. Logged in via putty; from that login, the system seemed to be running normally, but nothing would unwedge the display, including unity –replace
I gave up and rebooted, but the reboot process never changed the display; I had to power cycle the system.
dmsg -T printed messages like this one; they were in syslog after the power cycle.
May 13 13:18:20 desktop kernel: [185318.986092] nouveau 0000:01:00.0: fifo: CACHE_ERROR – ch 9 [firefox] subc 5 mthd 0180 data beef0301
(All things being equal, I'd just as soon use the open source software, but I just switched to the Nvidia proprietary drivers per the instructions here: Install Nvidia driver instead nouveau)