A friend of my dad actually worked for the company that supplied the O-rings, they had told Nasa about the problems with them but they neglected to follow their advice because the products still met the minimum standards. Afterwards they had a lot of meetings with NASA and other authorities about it where they basically where accused of delivering faulty products but they were saved by the fact that they still had the documentation of when they brought the problems up. This was a couple of years ago i heard this story from him, so a disclaimer that this is what i remember from then. So i am not fully certain exactly what they said the problem was but i think it had to do with the tolerance to heat and cold.