Must of got up on the wrong side of the bed.
Well, certain basic grammatical errors just jump off the page at me and are distracting.
I was taught that if you can't write clearly, you can't think clearly, and I believe that. "Must of" makes no kind of logical sense at all.
Plus, substituting "must of" for "must have" makes a writer look like a major looser.
(Ha-ha, you thought I spelled "loser" incorrectly, but I'm just messin' with ya.)