12 More Movies You Didn't Realise Stupidly Broke Their Own Rules

3. The Boggart - Harry Potter & The Prisoner Of Azkaban

The Rule:

The only way to defeat a Boggart is the spell Riddikulus, combined with a clear image of something ridiculous that undoes its fearful powers. The charm causes the creature to assume a form that is funny to the caster, and as laughter is their weakness, they are defeated.

The Breaker:

Because rules in Harry Potter are apparently only there to be broken, the Riddikulus charm is conveniently forgotten when Professor Lupin uses a Boggart to try to teach Harry to deal with his fear of Dementors.

During their training sessions, Lupin urges Harry to project a Patronus to ward off the Dementor, which would be all well and good if it actually was a Dementor. But it's not. It's a Boggart appearing as a Dementor, and as such using a Patronus Charm against it would be as effective as casting Reparo.

Luckily though, the film just ignores its own rule and Harry is able to heroically fight the DementorBoggart off with his Patronus. As if that's just fine to do.