It has happened on 2010-03-09 while I was playing Bob Came in Pieces in Windows. (nvidia-crash-2011-03-09.png)
Later it happened again on 2010-06-10, also inside Windows. (nvidia-crash-2011-06-10.png)
But last month, in 2011-11-14, it happened while I was inside Linux. I wasn't doing anything "heavy". Actually, at that exact moment I was switching around workspaces using Cube plugin from Compiz and I had very few open windows. (nvidia-crash-2011-11-14-001.png, nvidia-crash-2011-11-14-002.png, nvidia-crash-2011-11-14-003.png, nvidia-crash-2011-11-14-004.png)
The following message was sent to the system log right after the videocard crashed:
NVRM: Xid (0000:01:00): 13, 0003 00000000 00008297 00000f04 3f266666 00000040
The solution for these crashes? Rebooting. Actually, maybe it might be required to power off the computer and then turn it on after a few seconds.
I don't know why they happen, or how to avoid them, but at least they happen very rarely.