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Nightmare Fuel: Apollo 13
The prologue, showing the doomed crew of Apollo 1 banging on the hatch window, attempting to get out of the burning capsule. Then the glove goes back into the fire...
Marilyn's nightmare before the actual mission, which ended with Jim getting knocked into space.
The scene where Haise and Swaggert have started fighting. Not only are they using up more air (and producing more carbon dioxide) with all the shouting, but the prospects of a brawl in a ship whose hull is only a few millimeters thick is too horrifying to consider.