Should of, or Could of
When it's spoken, it really doesn't sound much different than the way it is supposed to be spelled. It's the written form that gets me.
Should've, or Could've
Leno had the duct tape kid on his show one night. The kid said the origin of the word was duck tape, and yes, I think it went back to one of the world wars. That's what he said; I have no proof.
The other one that gets me is, "supposably".