The Lunar Landing Training Vehicle was used during the Apollo missions to simulate flying the lunar lander at 1/6th the force of Earth's gravity:
Many sources have indicated a conflict between astronauts and management about the trainer - with the management deeming it dangerous and trying to get rid of it, and the astronauts wanting to keep it, calling it a vital training device.
It seems that astronauts had to bail out of failing LLTVs more than once - which would certainly cause me to think it's slightly dangerous.
Stuart Present ejecting from a failing LLTV
However, I've never seen anything which mentions the exact reason the trainer was unreliable - was it simply designed or built too quickly? Was there a vital design flaw? A certain piece of hardware that continually failed?