What movie did you think was "fine", and then the last twenty minutes made it 5-stars?
Inspired by me misunderstanding this post... https://www.reddit.com/r/Letterboxd/comments/1elad8j/what_movie_is_45_stars_until_the_last_20_minutes/
The first movie I think of is Incendies. A good movie all the way through, but after the "Can one plus one equal one?" scene, it got a spot on my four favourites.