Lu; what concerns me most is that 'O' ring erosion had been noted on several earlier flights.
Instead of realising that this was indication of a design problem, NASA appears to have concluded that it was safe to continue flying because previous flights had completed successfully in spite of the 'O' ring erosion.
But the 'O' rings were not designed to work in this way. Just because you get away with it a few times doesn't mean it is safe.
Now we have similar statements from NASA that debris from the external tank had damaged tiles before, and this was not considered a problem because the flights had completed successfully. Very worrying.