21: Funniest misinterpretation of a rule in your group?
Going outside RPGs to wargaming, and going back twenty-five years, the first time (possibly the only time) RM played BattleTech and we started to wonder why his lightly armoured Whitworth was shrugging off so much damage. The game uses a standard mech sheet, and the armour and damage tracks are represented by dots. And he was counting the lightly crossed out unarmoured dots rather than the blank armoured ones.
Still, good to learn this all together a year or so before Vampire: The Masquerade came out.