One thing that annoys me though this isnt table behaviour, is when a player says something like

Player:"Hey DM, you are wrong, Beholders dont travel in groups, they are exclusively solitary creatures."

Me: "I know. "

Player: "So why are they?"

Me: "Why dont you ask that in character?"

Player: "Because its stupid they dont do that. You are wrong"

Me: "No Im not wrong, they are illusions you imbecile! You dont think I read the paragragh on Beholder evology!"

Player: "Oh so they are illusions now. Bet they werent until I said they were."

Me: *Wishes this was an in person game so I could put a bullet between this A-hole's eyes"