When people say "veggies" instead of "vegetables". I don't know why, but it grinds on my nerves to no end.
Also "sexist" and "racist", because they are often misused. Example: I was watching a trailer for the Mortal Kombat Skarlet DLC. Someone in the comments section complained that there weren't enough female characters in the roster. So I said, (As a joke) "Well, 7 girls equals 14 boobs to 21 guys, so I don't think it's too unbalanced ;P " (I actually put in the little smiley face emoticon so they would know that it was a joke.) So this person sends me a reply calling me a sexist pig. I was not being sexist. Saying that among 7 females there would be 14 boobs is not a prejudice, it's a fact! I then replied to her saying, "Fine I'll be fair. 21 guys equals 42 testicles, so now it's unbalanced again. Happy now? " Surprise, surprise: she wasn't happy.
And lastly, I hate it when people use the wrong form of they're, their, and there. Knowing which one is right isn't exactly difficult.