This is a lot like that "Missed It By That Much" thread, except that most entries there are about scenes or moments you wish a story had, and most of mine are about that one scene or moment too many. (Obviously it's possible now to do "fan edits" and even publish them, but that's one more computer skill I've never learned.)
When it comes to very light stories, one of mine is that scene at the end of FATHOM, where the main villain is shot by Anthony Franciosa from behind, and he sees that streak of blood on the stolen item, and makes some dramatic comment about it before dying. That's a moment that could be just right for some very serious "Treasure of Sierra Madre" kind of treasure-hunting story, but I don't know what made them think it was right for this completely escapist one! It sounds really fanatical, but that moment is enough to keep me from buying that $4.00 DVD I seem to see in every discount bin.
Another one - and this is in a comedy that's generally dark, so I admit it isn't such a sore thumb - is that "Hawaii" sub-plot in WILD IN THE STREETS, telling how Max Frost did something about all the people in that state who voted against him. Somehow the internment camp part of the story makes sense whereas that part just seems forced.
(Also "Williger the crawdad"!)