Jump the shark – a term to describe a moment when something that was once great has reached a point where it will now decline in quality and popularity. Going downhill. (The Urban Dictionary)
My favorite part is at 4:09 in, where Richie says, “A shark! That is the stupidest thing I’ve ever heard… Fonz, you’re not jumping over garbage cans on a bike, you’re jumping over a shark!” The suspense crescendos at 8:35.