The AutoCorrect feature can be a problem if you regularly use a word that is not in the main dictionary, but which is nevertheless similar to a word present in the latter. To correct this problem, you can add the word to the AutoCorrect Exceptions list:
Open the AutoCorrect Options windows as described above.
Click on the Exceptions button to the right.
Select the "Other Corrections" tab and enter your word(s) in the "Don't correct field":