I’m heavily reliant on the Mac OS X spell checker, but I sometimes click Add to Dictionary instead of the correct spelling. For obvious reasons, that isn’t good, so it’s helpful to know where those new words are stored. Here’s an example:

You can view the custom dictionary by opening Finder and navigating to:

Users/USER_NAME/Library/Spelling/

In that directory, you’ll see an en file and a LocalDictionary file. You can open each of those files with the TextEdit.app and view the words that were added to the dictionary. Embarrassingly, my Local Dictionary file currently looks like this:

Delete any unwanted words and save the file. The changes will take effect once you reboot.

Current as of Mac OS X 10.11.6 (El Capitan)