I'm going to say a major pet peeve of mine is actually people getting too caught up on the grammar/spelling in an online post. When I see someone respond to someone's post just to make a grammar/spelling correction I find that incredibly rude, and if I were an admin/mod of a forum I would not tolerate it for even one second.
I still can't forget this one time on another forum where someone made a topic asking a question about some show and they misspelled a name or term from the show (which was not a word that was even common knowledge and had weird spelling IIRC) and the first member that responded to the topic had the audacity to be say "You misspelled [insert term here]. Until you fix that spelling I will not answer your question." Actually, I think it was even ruder than I'm conveying here.
I reported it and just thought "Get the *bleep* over yourself!".