Pulling an Air Bud

An unwritten rule in all sports rulebooks that states that a referee cannot call a penalty on a team no matter how ridiculous their actions are if there is not a rule in the rulebook that makes it illegal. Stems from the movie Air Bud in which a referee allows Buddy the Dog to play because there is no rule that says that dogs can't play basketball.

Coach: "He just threw dirt in my quarterbacks face to prevent his pass! Where's the flag?"
Ref: "Is there a rule saying he can't?"
Coach: "Well, no, but..."
Ref: "Than the play stands."
Coach: "Are you seriously pulling an air Bud right now?"