Put this one in the “You Learn Something New Every Day” category:
Apparently it is possible to have temperatures below absolute zero. John Baez criticizes this New Scientist article (in a g+ post – which I can’t figure out how to link to), but apparently the basic point isn’t completely wrong.
You can have systems with a negative temperature (and when they have a negative temperature they are hotter than when they have a positive temperature (!?)).
There’s a post explaining this on John’s website, so go read that if you want to understand what’s going on.