re: The Challenger exploded on this day 38 years ago...January 28, 1986.

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This shite had been going on since the git go and presumed to be an acceptable risk.

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A disaster almost happened in 1984 with STS-41-D (Discovery's maiden voyage), Mike Mullane (one of the crewmembers) notes it in his book Riding Rockets:

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But even as we scorched the prayer line with our pleas for flawless SSME function, both SRBs were betraying us. A primary O-ring at different joints in each tube had failed to seal as the motors had ignited. Tentacles of flame from the combustion area had wiggled between the segment facings. Like something alive and trapped, the gas had been wild to escape. It had reached the leak points and started to consume the O-ring rubber. The leak on the left-side SRB was bad enough for hot gas to actually get past the primary O-ring. Though we wouldn’t know it until after the Challenger disaster, we had just experienced the first case of what the Thiokol engineers would later define as “blow-by.” Hot gas had penetrated into the space between the primary and backup O-rings. Had our leak continued moments longer, the primary and backup O-rings would have been consumed and history would have recorded the Discovery disaster instead of Challenger. It would have been Zoo Crew’s names etched in an Arlington Cemetery monument. But the leak hadn’t continued. Inexplicably the primary O-rings had resealed.

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NASA was lucky as hell it didn't happen earlier (and 41-D wasn't the last mission a blow-by occured).