The case
Roy Scheider's ad-libbed line after the shark first surfaces is quoted with "we're" almost universally. In the film it's "you're," because Brody is telling the boat's owner about the owner's boat.
As a stand-alone joke the line works better in the first person, so that's how it's been used in headlines and sitcoms for fifty years.
Why our brains do this
Quotes migrate toward the form that works out of context. "We're" makes the joke about the speaker, which is how people use it.
Sources: Wikipedia — Jaws (film) · Wikipedia — False memory: the Mandela effect
