Or you really enjoy the film but a year or two later realize it's not really a keeper. Except for that one scene.
This is not to say the movie in question is no good; it just doesn't stack up against some of the all time greats.
So my definition of an essential scene from a non-essential movie is the sort of thing where you say to a friend, "Your life probably won't change if you never see the whole film. But you gotta see THIS bit!"
Herewith my initial list of essential scenes. Most of these are perfectly fine films. just not essential.
- - Superman saves the aeroplane in Superman Returns
- The monster appears for the first time, eats a whole lot of people and steals the girl in The Host
- Bart skateboards naked in The Simpsons Movie
- The pod race in Star Wars - The Phantom Menace. (Don't get me started on George Lucas's betrayal of all things Star Wars. If I had a DeLorean and could go back to 1985 to make sure he stopped after Return of the Jedi...)
- There's a 1 minute long scene in 300 where the camera tracks alongside the Spartan king during battle, with the film speeding and slowing along with his sword thrusts and spear throwing. Awesome.
- Motorway chase in The Matrix Reloaded. (Really, who still cares about those sequels?)