Simplifying a bit...<p>They knew the o-rings were being eroded away during the flight, but the secondary o-ring was &#x27;squishy&#x27; enough to fill in the gap and prevent the erosion from actually destroying the vehicle. While the erosion was unexpected, they figured the &#x27;backup&#x27; was doing its job, and actually ended up increasing the predicted safety margin (i.e. it&#x27;s only working 1&#x2F;5th of the way through, so we have a 500% safety margin, yay! despite the fact that any erosion was unexpected in first place)<p>The problem was, the cold made the secondary o-ring stiff enough that it didn&#x27;t &#x27;squish&#x27; as much as it had in previous launches, so the o-ring failed completely.