No special warnings and it's not even that deep, this guy just managed to get under my skin super quickly in a Pathfinder 1st edition Rise of the Runelords game. I'm going to call him Gary, we'd never met and the entire group was all online strangers assembled to play the AP underneath a new GM (I think the GM being new is why this guy wasn't kicked sooner).
It started almost immediately when I was talking about my build, I'd taken Divine Fighting Technique as a feat so that I could add my charisma to my starknife attacks as a Bard who worshipped Desna. Gary immediately hops in to tell me I can't take it as a regular feat at 1, I have to replace my Versatile Performance for it at level 2. I try to explain to him that this is an OPTIONAL REPLACEMENT (It literally says as much on the text line), that anyone can take this feat and that bards just have a secondary way into it, and he just absolutely bulldozes me in conversation to talk over me to explain how basically I'm stupid.
Eventually he gets off my back because I show messages from the questions channel in the Pathfinder Discord where everyone else made it clear as day that you can just take DFT as a feat, because it's a feat.
The equivalent of this would happen basically any time I did anything. By the time I was in this campaign (which was a few years ago now) I'd been playing PF1e for ten fucking years, I was not new, I knew how to play a level 1 Bard. He was only doing this to me and I was the only woman present, so I have to assume that was the reason, or maybe he was just mad I knew more about the game than him and wanted to try to neg me at every possible angle. I don't even mind it when people try to correct me but the fact that he was both smug & wrong just made him the most annoying person on Earth.
The straw that broke the camel's back was after a session when I was talking to another player about my build plan for my character, what i wanted to take at higher levels, and Gary just butts in to say "Do you even know how to have fun?" and I'm just fucking stunlocked because how do you even respond to that? What's wrong with this guy? So I just leave the call & the campaign afterwards. The GM told me he didn't last long afterwards but I was kinda out of it by that point, still possibly the most purely irritating person I've ever interacted with in a Pathfinder game.