My personal most aggravating form of cheat is the "roll-off guy".
There's a section in the rulebook that mentions that if the players disagree, the simple expedient of a roll-off can get the game back on track.
This player will then exploit that to the fullest by always misinterpreting rules.
Example: "You can't charge me, you ran in the shooting phase." "That's not how I remember, roll off! 4+ to charge?"
"You can't shoot all your guns, you moved full speed!" "That's not how I remember, roll off! 4+ to shoot?"
"You can't take both armor and cover saves, you pick the best one of the available ones!" "I don't think so, roll off! 4+ to get both saves?" "But I have the rulebook right here..." "NO WAY man, you're slowing down the game! It says to just roll off in these situations!"
etc., etc... And god help you if the rule actually is a more complex one. Its like he's trying to justify cheating by saying its a "4+" to do it. It makes it very tempting to try and play it back at him, ala:
"My LRBT Tank Shocks your Monolith. Its instantly blown away, cause I have a bulldozer. Oh, that's not how it works? Roll off! 4+ to instagib the Monolith!"
But then I would feel bad, lol.