Diane Vaughn wrote about 'normalisation of deviance' in her book The Challenger Launch Decision. The original Space Shuttle SSRB design had redundant O-rings for safety, and while a small amount of O-ring erosion (from the hot gases of burning propellant) had been observed on several occasions prior to 1986, it had never resulted in failure, so after a while, they began to accept it as normal. But as Richard Feynman pointed out, those O-rings had never been designed to erode. The first time erosion was seen was a deviance; it indicated that the protective thermal putty was not doing its job, and should have been corrected then.
