A few times now, my system just suddenly froze and stopped doing anything at all. The mouse cursor wouldn't move, keyboard shortcuts did nothing (not even ctrl+alt+backspace), nothing would respond, the screen was frozen too. I ended up pressing the reset button because it's all I could do. I've yet to try out the REISUB SysRq code, but next time it happens I'll try.
Obviously this is a serious problem, but I don't really know how to find out what happened, because I didn't see a kernel panic or anything. Is there a way to find out what happened afterwards, maybe by reading log files? Where should I look?