The words this week aren't homophones. But I felt compelled to include them because as I find writers absentmindedly interchanging them, it's become my second-biggest grammatical pet peeve*. These two words are completely different – they have different spellings, different pronunciations, and different meanings. They're simply not related. But what does that matter to the careless writer? They both pass spell-check, don't they?
* In case you're wondering, "must of" is #1.