I think "my bad" is a useful coinage.

What is "a$$hat"?

I think I am most annoyed by the following (partly because of their frequency, partly because of their egregious nature): -

"For free" in stead of "free" or "for nothing"
Split infinitives
"Checkout" in stead of "till"
"Janitor" (or, worse, "in-store janitor") for "cleaner"
"Regular" for "ordinary". "Regular" means "recurring with a fixed periodicity".
"Expiration" (which means "exhalation") in place of "expiry"
"In the event that" in stead of "if".
"Less" in place of "fewer" (the former applies to amorphous quantities, the latter to numbers of individuals or individual things).

I am confident that more will occur to me as the evening wears on and the bottle empties.