I have a huge problem with grammar mistakes. Oddly enough, though, it is only certain ones. There/they're/their is distracting to me, and to/two/too trips me up as well. Improper usage of the perfect conditional (he/she would have <rest of sentence>) drives me crazy.

"I wish I would've done better in school." "I wish he wouldn't have died." No. You wish you had done better in school. You wish he hadn't died.

It also drives me nuts -- and I will correct people in the middle of talking -- when people use regular past tense where they should be using the past participle.

Interestingly, the above two issues didn't bother me until I took Spanish.

The two things that bother me currently in writing nowadays are:

* "would of, could of, and should of" instead of "would've, could've, and should've"
* using apostrophes to indicate plural (but not possessive or contraction)

My English teacher last year once saw a sign that said "USED BIKE'S FOR SALE", and was annoyed enough to go up to the store owners to correct their mistake.

But, as I stated, they're annoyances. Some of them are really distracting to me, but all-in-all, they're annoying. It's especially annoying when ESOLs have better grammar and spelling than native English speakers.