Simplifying a bit... They knew the o-rings were being eroded away during the flight, but the secondary o-ring was 'squishy' enough to fill in the gap and preve

Simplifying a bit... They knew the o-rings were being eroded away during the flight, but the secondary o-ring was 'squishy' enough to fill in the gap and prevent the erosion from actually destroying the vehicle. While the erosion was unexpected, they figured the 'backup' was doing its job, and actually ended up increasing the predicted safety margin (i.e. it's only working 1/5th of the way through, so we have a 500% safety margin, yay! despite the fact that any erosion was unexpected in first place) The problem was, the cold made the secondary o-ring stiff enough that it didn't 'squish' as much as it had in previous launches, so the o-ring failed completely.