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Nightmare Fuel: Walrusguy
"Zelda Fails at Giving Tutorials". It lasts less than 10 seconds, but boy does it use that time well.
For those who don't want to Google it, the video is the first half of Zelda's tutorial in one of the infamous The Legend Of Zelda CDI Games. But then midway through she suddenly stops, bulges in a VERY ugly looking way to a very disturbing moan, then explodes.
We should note how the gem on her headband becomes an upside-down cross when her face morphs.
Gene Deitch's The Land Before Time has a really weird and disturbing moment when one of the dinosaurs (which species seems to have been made up by Dingo Pictures) has his dialog replaced with a loud, metalic sound. Oh, and at the very end, one of the dinosaurs Makes the same sound as Zelda in the aformentioned "Zelda fails at giving tutorials".
The end of A Rabbit Puppet Traumatizes Some Schoolkids has loud metal and disturbing shots of Doogan brushing his teeth, ending with the baby from Eraserhead.