I have lived through some lock ups in the recent past, but that's because I've disassembled my desktop from it's case and assembled it at my working table. Since both PS/2 ports of the mobo are on my mouse pad (yeah, short cables, tight space), I eventually pull some cable os slap my video card. The first time X.org locked up, but ssh'ing from my Maemo phone did the trick (had to disable kexec and reboot from the bios, though). The second time I couldn't do that, my video card cooler fans stopped, and I had to pull the power plug and wait a few moments. That time I thought that some money would be spent on a new motherboard or video card. But rest assured, no computer components were harmed in those accidents.
Ending the off topic, I had lock downs and headaches when messing with reiser4. I'd say that recently built experimental code in the kernel is something that would trigger lock ups.
