I was sure I'd already answered this one, but digging through my archives, I haven't. It's an interesting problem to address, though, because there's the ridiculously hard and dangerous solution and then there's a surprisingly easy path to unlearning a mispelled (uh, "misspelled") word that's available in a bunch of Mac OS X applications already.

The hard, complicated way you'll find if you Google for a solution and go somewhere other than Ask Dave Taylor, of course. It involves you going to your Library/Spelling/LocalDictionary and editing the file using a Terminal-based text editor. Way too much work, and the danger of accidentally wrecking the format of the file is too darn high.

Instead, it turns out that there's a simple mouse click that does the work for you!

To demonstrate, I'm going to use TextEdit (it should work in other apps, but doesn't seem to be part of the current version of WebKit so it's missing in Safari, Apple Mail, etc. No worries: if you just live in Mail, you can still launch TextEdit, fix your dictionary errors, and it'll be changed system-wide.

Let's start out with a misspelled word in TextEdit:

Obviously, I have terribly misspelled the word "banana" in this sentence, and TextEdit has correctly flagged it with the squiggly red underline. A quick Ctrl-click to bring up the context menu and:

I'm going to "Learn Spelling" and...

Now you can see the word isn't flagged as being incorrectly spelled by the Mac OS X spelling dictionary. Problem is, we actually do want it to be flagged because, well, that's not the correct spelling.

So here's the secret: Ctrl-click on the correctly spelled word that's now in the spelling dictionary and check out the context menu:

If you're yelling "pick Unlearn Spelling, pick Unlearn Spelling!" you're right!

Now check it out:

The word is correctly being indicated as misspelled by the spell check system on the Mac again. Phew. Problem solved, and no complicated steps are involved!