Since December 1982, the O-rings had been designated a “Criticality 1″ item by NASA, denoting a component without a backup, whose failure would result in the loss of the shuttle and its crew.
Richard Feynman[:] “… [the shuttle] flies [with O-ring erosion] and nothing happens. Then it is suggested, therefore, that the risk is no longer so high for the next flights. We can lower our standards a little bit because we got away with it last time. You got away with it, but it shouldn’t be done over and over again like that.”