Pet peeves:

"Same difference."
It's NO difference, not same difference when you're not stating that the difference between A and C to B is the same. Morons.

"But yet."
It's either just a "but" or "and yet". To say "but yet" is a like using a double negative; you don't say "but but" or "yet yet" so please, why a "but yet"?

"Could of sworn" or something similar.
"Could have sworn" not "could of sworn". Dumbass. This, of course, pokes at myself.

"Ain't know nothing" or "ain't got nothing" or something along those lines.
Unless you're a Southern do-hickey or the author of a story dealing in undereducated characters, one really should know better than to make such an illogical statement when meaning the opposite. It's "ain't know anything", dipstick.

The mixing up of "your" and "you're" in written work. "Their" and "They're"; "breath" and "breathe", "Where" and "were" and "we're".