One thing that's not safe is leaving your computer halfway through an apt-get upgrade which gets interrupted by an error or for some other reason. I did this, and X started okay, but once it was into GNOME the computer started sending all kinds of random keystrokes to the gnome-terminal I had running. I tried to move my mouse to close it, but the mouse was all fucked up and the pointer jumped around and sent random clicks. It managed to open up mutt and do random things to e-mail messages as I tried ctrl-alt-backspace, which didn't work. And I've got a stupid keyboard which goes catatonic when I press Alt-SysRq. I had no choice but to hit the power button.

Finishing the apt-get upgrade solved everything, but this goes down as the most impressive complete crash of Linux I've seen.