Mini Rant/Vent

So, I'm reading what's been a totally enjoyable, clever fan fic about the latest couple to capture my attention.  Suddenly, there IT is -- my number one grammatical pet peeve, like fingernails down a chalkboard and I'm thrown out of my enjoyment of the fic.  IT being "could of" -- I don't know why, but a couple of spelling errors or maybe misuse of there/their/they're or then/than and I can note it and move on.  But throw out could of, would of, or should of and I come to a screeching halt, have to roll my eyes and almost force myself to keep reading.