You may have done what I did, and added a misspelled word to the built in spelling dictionary of your Mac. This could be a problem, since it wouldn’t find this word later. The problem is, there are no UI for managing these words (not one I know of).
The words you made you spell checker learn are stored in a simple text file in the Spelling folder of your user Library. From Snow Leopard and up you can find this file in the following location:
On older systems, there are language specific dictionaries, for example:
After finding the file, you can edit it with the text editor of your choice.