Feynman, 1986: (second paragraph)
We have also found that certification criteria used in Flight Readiness Reviews often develop a gradually decreasing strictness. The argument that the same risk was flown before without failure is often accepted as an argument for the safety of accepting it again. Because of this, obvious weaknesses are accepted again and again, sometimes without a sufficiently serious attempt to remedy them, or to delay a flight because of their continued presence.
...don't know how many tiles, can't respond to that hear, today, have no capability to repair tile, only recourse to design so that we don't lose tiles, so we can take impacts without it being a safety concern, we have lost tiles before on bottom of vehicle, have had debris impacts before, they have all been acceptable and don't represent safety of flight concern, would like a harder tile, but it has not to date presented a safety concern and have no recourse if we lose tiles, only effective course is to prevent loss through design and test, and has been perfectly adequate to this point.