That seems to be the classic punchline that doesn’t even need the setup, and it’s been used that way in two movies that I know of.
In Men in Black, Tommy Lee Jones zaps Will Smith with the neuralizer. When he comes to his sense, they’re sitting in a restaurant, Jones is laughing hysterically (apparently right in the middle of telling a great joke) and that’s the punchline.
Similarly in The Sting. After his big score early in the movie, Robert Redford goes to see his girlfriend who’s performing in a burlesque show. When he arrives, a comic is on stage in the middle of his act, and that’s the punchline we get to hear.
The scene where this happens in the 1982 American film Diner is https://www.youtube.com/watch?v=vujNcE_43Qg. In the comments section on that YouTube video, someone suggests that the scene was copied from the 1980 French movie La Boum. The scene in that movie where the same thing happens is https://www.youtube.com/watch?v=jl-8OXJX0J8. There’s just one problem with that suggestion. Although La Boum was released in France in 1980, according to the IMDb entry on that movie, it wasn’t released in the U.S. until 1983. So if Barry Levinson, the director and writer of Diner, copied it from La Boum, he would have had to have seen it on a visit to France (or one of the other countries where it was released before he wrote Diner).
As a matter of dating the legend, I heard of it being done (by a friend of a friend, naturally) during my first summer at Boy Scout camp. Camp was concurrent with the Woodstock Festival (and less than 20 miles away) which puts it as early as August 1969.
This must be the dirty joke that Tommy Lee Jones told to Will Smith in Men in Black, but we only saw the tail end of it because Smith had been neuralyzed.
Most people claim urban legends as happing to a friend of a friend, or even to a friend. It’s interesting when they claim to have personally experienced or did it.
Urban Legends are things that are implausible. Do you really think that no teen boy has ever stuck his wiener through the bottom of a box of popcorn (when that box had a pre-made opening exactly over said wiener)?
So not true. Many/most have outsized elements like a murderer with a hook for a hand looking for cars on Lover’s Lane, gang signs flashing headlights before they kill you, kidnapping toddlers from stores by hiding them in a stack of tires/cutting off their long hair in the restroom, Slenderman, winning a million bucks by sending chain emails, cats stealing the breath from a baby, choking a baby to death by place a few grains of rice in the back of their throat, etc.
There’s no way to “prove” the movie theater popcorn box trick without calling in a news crew to film it.