The Forget button will remove any custom words that you added to your Mac’s spelling dictionary, as long as you can remember the word that you misspelled.

I was recently editing a document in Pages when I came across a misspelled word. Instead of Control-clicking and selecting the correct spelling, I accidentally clicked Learn Spelling. How can I delete the added word? I went into Dictionary.app and couldn’t find my misspelled word there.

You won’t find your “custom word” inside the Dictionary application, because custom words are kept in supplementary spelling files that are categorized by language at ~/Library/Spelling.

If you want to see all of the custom words that have been added to one of these supplementary spelling files, simply open the file in a text editor that lets you view invisible characters, such as TextWrangler (free, www.barebones.com). You’ll see your words, separated by an invisible red ¿ character, and you can delete your custom words from there. Save the file and log out of your account for the changes to take effect.

However, there’s an even easier way to delete a custom word, if you can remember the word that you misspelled. Simply launch Pages (or any other program that uses the Mac’s built-in spelling dictionary), go to Edit > Spelling > Spelling, and type in the word that you misspelled. Then click the Forget button.