Most simple grammar doesn't bother me, like the error Keyser and myself made by placing an apostrophe where there shouldn't have been one. What does annoy me are these being used incorrectly:
your, you're
there, they're, their
where, were, we're (I find smartphones constantly balls up we're and were though)
"was" being used where "were" should be
I am far from perfect with my spelling and grammar by the way, so anyone who mixes ^ those ^ up, please don't be offended