I think someone already said this one, but using "literally" when they mean "figuratively."
My wife would love this thread. Poor grammar drives her nuts. When we got married, I (unbeknownst to me) had picked up a few coloquialisms that she couldn't stand, and after repeatedly correcting me when she caught me saying them, have excised them from my vocabulary. Among the worst offenders:
"Nother." As in, "I'm a pretty good skater, but rollerblading is a whole 'nother thing."
"One of them." I think this might be an east-coast thing. "What 'cha got there, Tom? Is that one of them new Camaros?"
"This/these things." If I'm telling my wife a story and say something like, "I was at the bus stop, and I met this homebrewer..." she'll cut me off and ask, "Which homebrewer? This one?" (Pointing out into space, feigning pointing at a person) "Or this one? Maybe this one? Or did you mean you met a homebrewer?"
"Some good." As in, "I had a slice of that birthday cake, and man, it was some good." My wife will ask, "Just some good? Not all good?"
"Right nice." I never really said this one much, but my grandmother says it all the time. "I was at church last week and they've finished rebuilding the pews. Now they look right nice."
I'm sure there's more, but those ones stick out in my mind.