I do believe this is the first time I've ever cited Readers Digest in a blog post. But, you got to give credit where it's due -- they've put together a list of "24 Things You May Be Saying Wrong" which is a must-read for those who don't want to unwittingly seem half-witted.
For instance, "mano a mano" doesn't mean "man to man", i.e., a man to man talk; instead, it means "hand to hand" as in "mortal combat."
The article also shines some light on the "who/whom" debate, "further/farther", "affect/effect", "complement/compliment", as well as running the entire gamut from "less/fewer" to "more than/over".
Other highlights for me? That "supposably" is actually a word, although it means "conceivably" and not "supposedly" (the correct version for the usually-intended meaning). Well worth a quick read, especially if you suspect that you might be making some common mistakes. (Which, since they're "common", means most of us!)