Comment 4 for bug 553237

Hm.. thanks, I can't confirm it anymore! Give me 48 hours (2 days) to check if this happens while leaving the computer on. I really hope this is just a one-time bug.

I don't understand completely how it works, but I think it doesn't happen to all machines. I'm not the only one that has this problem -- 2 other people have confirmed the same problem.
You can see the bug report at debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=557949