It was still a bad O-ring...that was known at the time before launch.
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But what really brought down the Shuttle was sheer bad luck. If the burn-thru had happened anywhere in the 270-degree arc facing away from the ET, the Shuttle would have made orbit with no problems, even with the burn-thru.
Only the bad luck that the burn-thru was facing inwards towards the ET and the mounting brace was the problem.
Well known before launch. I know from Bechtal engineers involved in the testing of the solid fuel booster rockets.