Ah, the dreaded homonym. Spell checkers are no defense, either; you have to actually LEARN the differences. So few do these days and that's why we have people pouring over books, breaking their cars to a stop, reigning their emotions in, etc. The worst for me are horrible bloopers involving the words affect and effect. Homonyms truly can change language meaning and can even be dangerous in situations where clarity is required, like wars, explosives factories, etc.