I'm sure that for every good idea that made it into a spaceship design, there had to be plenty of bad ideas. There's most likely a bunch of old designs (or practices) that passed previously, but would be scoffed at today by our new standards. I'm mostly interested in the older designs that were used in space exploration that would be immediately declined today due to any number of reasons. Anything that was implemented/used in previous space exploration that would be absolutely banned by standards today would be a fit for an answer to this question.
For instance, lets assume we were in the 1980's attempting a new chemical composition of solid rocket fuel that ended up releasing 2-3 times the amount of acoustic vibrations than was expected.
Answering the following: "Why would we never have let this happen today, and why did we let this happen then?" Would be a great answer. I would like explanation of the incident to be in detail explaining:
Why it failed. What new regulations were put in place because of it. How we would do it now.