Grammatical mistakes in general annoy me.
From apostrophe misuse, to poor spelling, to incorrect punctuation, to incorrect word choice; honestly, I don't see how it's so hard for people to get correct.
The greengrocer's apostrophe is another favourite: people putting an apostrophe before the 's' in plurals, e.g. apple's. For God's sake, it's simple stuff, yet it's seemingly so difficult for some.