People on Dueling Network that never read a card's effect thoroughly and then argue that you're not using the effect right. For example, When Beelze, King of the Dark Dragons attacks and your opponent activate Magic Cylinder; you take the damage, then Beelze gains 3000 ATK. This one idiot claimed that it only works with battle damage, so I kept telling him to read the full effect, which only made him rage-quit. That's what really grinds my gears.