This is based on opinion. Please don't list it on a work's trope example list.
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. In real life, perhaps mercifully, it took only 15 seconds for the astronauts to asphyxiate, long before any burns to their bodies.
Marilyn's nightmare before the actual mission, which ended with Jim getting knocked into space. (She recently saw the film Marooned, depicting a stranded crew in space.)
The scene where Haise and Swigert 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. Typically, astronauts are conditioned with Nerves of Steel and training to avoid or weed out personality conflicts. Justified Trope: Carbon dioxide poisoning can, among other gruesome things, cause erratic behavior. Of course, Rule of Drama also applies (Lovell states in the commentary that the scene is entirely made up, and they never actually had any kind of fight).
There is also a Blink-and-You-Miss-It part in the scene where the crew is building the makeshift carbon dioxide filter. A quick point-of-view shot from Jack Swigert shows him getting tunnel vision from hypercapnia.
In-universe, Jim's youngest son is scared of his father going into space, because of the disaster on Apollo 1. His father assures him that the hatch problem that killed the Apollo 1 astronauts has been fixed and can't cause problems anymore. Then he is told that there's been an accident on Apollo 13, he calmly asks, "Was it the door?!"
A Real Life consideration: You may thank NASA engineer John Houbolt, indirectly, for Apollo 13's survival. Early in the planning stages, NASA planned on Earth Orbit Rendezvous, using two rockets to rendezvous around Earth to form a monolithic single lunar vehicle. In short, all three astronauts would land on the moon in this one vehicle. Houbolt, after reading about the concept of Lunar Orbit Rendezvous, as developed by a Russian scientist, made several attempts petitioning NASA that EOR would make the lunar vehicle too heavy, requiring more rockets and more energy to develop. While LOR had its risks, NASA eventually agreed. They designed two independent spacecraft: One an orbiting base with an earth-return command module, and a lunar vehicle designed only to land on the moon and return. Both vehicles had independent life support, propulsion, and guidance. As it turned out, NASA realized this advantage in case of an unlikely failure and had developed Lunar Module-as-lifeboat scenario training.note Some elements of the rescue plan had actually already been practiced. During Apollo 8, Jim Lovell accidentally deleted part of the Guidance Computer memory, requiring him to take star sightings and manually reprogram their position into the computer. On Apollo 9, the Lunar Module descent engine was fired while still docked to the Command Module, in the same manner as the course correction burns on 13. No one took it seriously until Apollo 13. If Earth Orbit Rendezvous were the choice, and similar problems occurred with the oxygen tanks that exploded, that single-vehicle idea would have utterly doomed this crew.
Similarly, if the explosion had occurred after the landing rather than before it, the Lunar Module would no longer be attached, leaving the astronauts with no option other than to hope they got back to Earth before the batteries and oxygen supplies ran out. In such a scenario, the crew likely would not have returned alive. Subsequent Apollo missions equipped the Service Module with much larger rechargeable batteries, just in case the fuel cells ever failed again.