After reading through some more of the responses, I thought of a few more commonly misconstrued words/phrases:
1) I often see people using the term "to cast dispersions" when they mean "to cast aspersions", as in: I'm not participating in this thread to cast aspersions -- it's just that language and grammar are of interest to me, and I enjoy reading about the quirks and foibles that go along with the process of communication.
2) It's always "its", unless it's a contraction for "it is".
3) I infer from what you imply, not the other way around.
4) This has already been noted, but it bears reiterating: When used as a verb, "affect" means to change, while "effect" means to cause.
5) It's a chaise longue, not a chaise lounge.
This also reminds me of one of my favorite stories from grad school. I was in a class in which the professor was trying to impress upon us the meaning of the phrase "moral turpitude". (I guess she had some insight into my character from the beginning.

) Anyway, to get some class participation she called on one of my friends sitting next to me, an amiable Southern good ol' boy who was prone to daydreaming in class. He looked startled when she said his name, but he was always quick-witted, so when she asked him if he knew what "turpitude" was, he didn't skip a beat:
"Yes, ma'am. It's something you mix paint with."
The rest of the class went downhill from there.