It annoys me when people mix up homonyms or pronouns, especially in professional publications or broadcasts:
whose/who's
there/their/they're
your/you're
her/she (I have heard these mixed up on E! news)
him/he
me/I
vein/vain
reign/rein/rain
e.g. "Her and me went to see a movie" should be "She and I went to see a movie".
The confusion with "I could/couldn't care less" is easily cleared up by simply saying, "I don't care".
Adding apostrophes where they don't belong makes me mad.
I have seen this on a menu: "Hamburger's - $5"
or
Those dog's are annoying!
The phrase 'whatnot' is annoying with overuse.
"So, I went to get my hair done and whatnot. Then I bought some skincare stuff and whatnot."
Another peeve is when grammar is misspelled as grammer.
Also: 'hisself' is not a word. The correct word is 'himself'.
One more thing...
would of
could of
should of
might of
^ These are all wrong.
The correct term is "would have" which can be contracted to "would've," which sounds like "would of," but should never be written that way.