In the truest definition of "pet peeve" I'd have to say I maybe have one that comes to mind and that is:

Writing in first person. And by that I mean my partner expecting [i]me[/i] to do it. I do not like it. It [i]bothers[/i] me and I knowww it's irrational, but I cannot explain to you why I HATE it I just. I do. Reading posts if that's how my rp partner(s) prefer to write? Totally fine, I can deal. But I am begging you don't make me write in first person too I will cry or spontaneously combust /joking XD