DnD has a weak one: wizards take fireball.
Gurps has a hard one:
1) Setup attack
2) Evalute setup success
3) If the rest of the team is attacking, attack anyway.
4) If the rest of the team isn’t attacking, setup attack.
Exalted had, like, fifty of them, resulting in cascading paranoia combos. And then third edition happened, but no one cared.