Certain common grammar mistakes drive me insane.  If I hear someone say, "should have went", I get a palpitation.  Then, there is the written choose/chose, lose/loose, their/they're/there.  A certain friend of mine will write "we could of won" instead of "we could've".  My twin will never get then/than sorted out.
I ain't no perfect grammar guy, but those things drive me batty.