It happens just when you don’t want it to – you press the wrong key and all of a sudden you’ve saved an incorrectly spelled word to your dictionary. Unless you fix the problem, Word won’t pick up this misspelling ever again.
To solve the problem, choose Tools, Options, Spelling & Grammar and click the Custom Dictionaries button, click Custom.dic and click Modify. Locate the misspelled word, select it and click Delete.
Now click Ok and you’re done.