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Nightmare Fuel: Indiana Jones and the Last Crusade
Donovan's rapid aging after he drinks from the wrong Grail. "He chose poorly", indeed. What's even more terrifying is that he's still attacking Elsa even after he's been reduced to a skeletal corpse!
The entire sequence leading up to that moment was pretty freaky, really. People's heads plopping down the steps at the beginning of the tunnel with no indication of what exactly caused it, the blades that come whipping out of the walls when you can't see anything for the cobwebs, the invisible handspan-wide bridge over the bottomless crevasse...following a film that up until then pretty much tried to be more kid-friendly than the previous two Indy movies, that was some serious Mood Whiplash.
If you're ophidiophobic, the sequence where Indy gets his famous fear of snakes will likely make your skin crawl.