Some words don't sound like what they mean. And some words don't mean what they sound like. We aim to provide some clarity.
could have/could've - not could of. Ever.
enormity (n) - despite how it sounds, enormity doesn't refer to size. Rather, means "extreme evil" or "a very evil act." The enormity here is how many times you'll hear this word used incorrectly.
histrionics (n) - Is there anyone left old enough to remember when John Tesh provided color commentary for Olympic gymnastics? Is there anyone old enough to remember John Tesh? Well, anyway... he said "histrionics" thinking it was a fancy way to say "history." It was hilarious. Histrionics means exaggerated emotional behavior. Which, when you think about what sometimes happens in gymnastics, isn't all that far off the mark.
insure/ensure - We've seen this wrong in the New York Times! Tsk! Insurance with an "i" always refers to the kind of insurance you have to pay for. You can remember this by saying, "I pay for insurance. I do. I do. Aye, aye, aye. (Smack forehead) It's expensive." Ensure means to make sure something happens. For example, we would ensure that we never get in a fender-bender. (And if insurance could do that, it would be worth what we pay for it.)
irregardless - Regardless of whether this appears in the dictionary, this is not a word. Not a real word, anyway. The dictionary calls it "non-standard," which is a polite way of saying "made up word used by people who don't know any better." The equivalent, as we have explained in our Encarta writing, would be wearing a pair of underwear as a hat. Sure, it might fit nicely on your head. But you'd look silly — almost as silly as someone running around saying "irregardless."
it's/its - It's with an apostrophe is short for it is. If you can substitute it is in your sentence, then you want it's. If you can't, then you want its, which is a possessive. Remember, his and hers don't have apostrophes, so its the possessive doesn't get one, either.
For the truly picky:
Is it compare with, or compare to? It depends on what you mean to say. If you're pointing out similarities, you compare with. If you're focusing on differences, then you compare to.