I know someone who says ‘subsequently’ when then mean ‘consequently’ and vice versa. It doesn’t sound like much, but they also overuse the words so friggin’ much that it drives me nuts.
My pet peeves are ‘definately’ and ‘loose’ (for ‘lose’- already mentioned), and failing to put the final ‘e’ on ‘breathe’ when using the word as a verb. (Gasp! I can’t breath!)