The Apollo 1 fire occurred, taking three of our Astronauts on their final voyage.
Gus Grissom, Ed White, and Roger Chaffee perished in the fire inside thier Apollo Command Module.
The cause of the fire was eventually traced to some chaffed/frayed wire that caused a spark, resulting in an extremely hot flash fire accelerated by the pure Oxygen used at the time in the Command Module.
In the race to beat the Russians to the Moon, compromises had been made in the design of the Command Module, including the use of a pure Oxygen atmosphere. At the time it was deemed too expensive, and more importantly, too heavy, to include the required Nitrogen system so the the atmosphere in the Command Module would be "air", and a much simpler pure Oxygen system was used.
The hatch on the side of the Command Module had also been designed to open inward, and had many bolts to secure it closed, making it very difficult to get into the Command Module quickly from the outside.
And many materials used in the interior of the Command Module were later found to be extremely flammable in a pure Oxygen atmosphere.
Manufacturing carelessness also played a factor in the accident, as quality controls were not up to the standards used today. One of the things that NASA did with the remaining Command Modules in the inventory was to put them on shake tables, and shake the living daylights out them to see if anything was loose. One thing I remember from from my NASA Soldering Certification classes was that several POUNDS of loose solder bits and loose solder "balls" were shaken loose from the Command Modules that were tested.
Every one of these little things was a potential short circuit that would be floating around in the microgravity experienced by the Command Module during it's flight, and the Engineers were horrified to see how much junk came out after the shake tests.
Wiring was also rerouted, tied down, and encapsulated with tubing at any point where it rub against anything to ensure that the insulation would remain intact.
And the pure Oxygen system was replaced with a system that used a 60/40 mix of Nitrogen and Oxygen until the Command Module was in orbit, at which time the Nitrogen was slowly purged, and the atmosphere replaced with pure Oxygen.
Manned spaceflight is an extremely risky business, even when practicing on the ground, and you will lose flight crews. You take every precaution you can, design things with triple redundancy, have detailed checklists that must be rigorously followed, and still, you cross your fingers and say a little prayer when the crew boards the spacecraft.
Man is a curious, exploring animal, driven in his quest by a thirst for knowledge and understanding.
And brave men like these lead the way.
God speed, Apollo 1 crew.
Per Aspera Ad Astrum