I always double-check things.  It's kind of an annoying habit that comes partially from this paranoia, but I can't help it.  I double-check that my homework is done, that my makeup looks good, that I'm wearing my spankies under my cheer skirt...  Usually I know that I don't need to, but I get really nervous if I don't.
  Then, of course, I double-check tests.  This is in no way an annoying habit, but one that is a life-saver.  Today I caught three or four mistakes on my math test before turning it in, working to the bell.  Which is great, because it seems like no matter how careful I am, the only reason I do bad on math tests is stupid mistakes.