10 Times The Simpsons Broke Its Own Rules
9. Bart's Shrimp Allergy
The B-Plot of Season 18's Please Homer, Don't Hammer 'Em pits Bart and Principal Skinner against in each in a battle of allergies.
This all begins when El Barto discovers that his nemesis is allergic to peanuts and goes around wielding one on a stick to force Skinner to do his bidding. To even the odds, Skinner steals Bart's medical records and then creates a stick of his own, this time with a shrimp on the end of it.
The problem with this - aside from the countless violations of basic human ethics that are involved in this story - is that Bart has eaten shrimp before.
He mentions that he ate some in Season 13's The Frying Game and has been around his father devouring the crustaceans on numerous occasions. With an allergy so sensitive that it can be set off by a single shrimp on a stick, you'd have to think Bart would have avoided them at all costs.
Side note - Bart is also allergic to butterscotch, imitation butterscotch, glow-in-the-dark monster make-up, and cauliflower. So Skinner could have put any one of those on a stick and still kept within the rules of the show.