Let´s talk about first piloted mission of the program Apollo. Before this one there were other missions but without people (AS 201,AS 202,AS 203). Apollo 1 was planned for 21st February 1967 but it ended before it could even start.
There were three guys in this mission
Virgil Ivan Grissom, Edward Higgins White, Roger Bruce Chaffee. All of them were experienced pilots from planes or Gemini missions… Grissom was captain.
So in 27th January there was one of many tests while this one was essential for leaving Earth month later.
Main problem with this mission was that it was made in great hurry so that NASA could finish goal made by president Kennedy. Guys who made cabin did that with flammable materials and NASA knew that. Problem was that it would need complete rebuilt of whole cabin to repair those problems and lot of money. So they decided to not to do anything with that.
All three members of crew knew that there are some mistakes and they were trying to point them out. Even that day of 27th January it was planned that Joe Shea will be present in this test inside rocket but he refused saying that he can watch from bunker. Slayton (guy who was candidate for astronaut) said that he will be there but finally he was not because there was not much free space inside.
Start of test had to be delayed because crew sensed some strange odor. Some guys checked it and it seemed that nothing is wrong. Cabin was full of pure oxygen with 110% atmospheric pressure which was purposeful to see if there are any gaps in cabin. Anyway you can probably see what happened now. First they had problems with radio connection and Grissom was saying something like this: “How can we land on Moon if we are not able to talk across three rooms!” Than some spark around Grissom´s seat made fire and it took five minutes before under-pressured cabin could be opened which was too late.
This was huge wound to NASA because it was first missions with people and it did not even started. Luckily next time it was much better prepared.