“Former astronaut Sally Ride says before Columbia exploded, the U.S. space agency may have gotten too used to foam breaking off the fuel tanks and striking the shuttle’s thermal protection system. | Investigators believe hard foam insulation falling from the external fuel tank during launch may have damaged the shuttle’s wing. The spacecraft broke apart upon re-entry into the earth’s atmosphere on February 1. Ms. Ride also says before the 1986 Challenger accident, NASA accepted problems related to the O-rings that connect the shuttle’s solid-fuel rockets with its external fuel tanks.”

—NASA Complacency Cited In Both Shuttle AccidentsVoice of America)

According to risk-assessment experts, people who work in risky environments tend to redefine the level of risk that is acceptable, and thus tend to ignore evidence that suggests a particular new event may elevate that risk. Thus, even if the breaking foam or the eroding O-rings were seen as clear problems by some engineers, the fact that for so many launches the observed problems didn’t cause huge disasters, those problems became invisible.