Space Shuttle Challenger disaster
Blow-by of the O-rings had been seen on early Shuttle flights, and a pattern was noticed by the engineers who built the boosters: more blow-by happened in cold temperatures. The company that built the boosters, Thiokol, had been working on a fix but had not told NASA even though the O-rings were "criticality 1" parts, meaning that if they should fail to do their job, the rocket would be destroyed. Criticality 1 parts were also not allowed to rely on a backup part for safety - this meant that the backup O-ring in the booster was not good enough to keep the astronauts safe.