The Mark 14 Torpedo debacle.
So, the Mark 14 was the main submarine-based torpedo of the US Navy in WW2. However, it was probably one of, if not the worst torpedo ever put in service, at least at the start of the war.
How so? Well, it had a number of disastrous faults.
Torpedo depth sensors. In a nutshell, the torpedo ran far deeper in the water than it was set to; this was partially due to terrible testing procedures (in testing, the ballast used was far lighter than the depth of an actual warhead) and a mistake which I am genuinely stunned was not noticed by anyone at the time; the sensor was located at the tail, in the low pressure wake of the torpedo. This all combined to make the Mark 14 run about 10 feet deeper than was intended, which led to a lot of torpedoes running straight below their intended targets.
Magnetic detonator faults. The magnetic detonator simply did not work; due to poor calibration, it tended to either fail to detonate when the target was directly above, or detonated far too early.
Contact detonator faults. Somehow, even the contact detonator was faulty as well; in one notable instance, USS Tinosa fired 15 torpedoes at a 19,000 ton whaling ship, and despite scoring 13 hits, the whaler survived. Nearly all of the torpedoes were duds, despite scoring textbook hits on the whaler.
Despite these faults, the US Bureau of Ordnance insisted, for several years, that their Mark 14 torpedo was absolutely perfect, and that every single US Submarine Commander was apparently a incompetent buffoon. Worse yet, they actively forbade any firing tests on the Mark 14, and went as far as recommending disciplinary action against a few submarine commanders who tried to work out of themselves what the hell was going on.
It took many years before BuOrd finally admitted they'd screwed up, and allowed fixes to be implemented.
People like to say that the US has always fumbled in history and it's mostly true, however the US was on the fucking ball over Thalidomide. It was legal everywhere else and considered a miracle for a little while in the 50s but while other countries rushed through its approval process, the FDA took its time to conduct research.
It wasn't until the 60s that the horrors of the effects of Thalidomide on unborn babies was known. It wasn't as if the FDA wasn't being pressured into approving it all that while, but they knew that the world didn't know enough of its effects to truly gauge its safety. They were being bribed, threatened, and harassed all that time.
While it's well known today that Thalidomide shouldn't be used during pregnancy, there was a time it was actually considered foolish of the US to hold up the process of this "miracle drug." You can say what you want about the FDA expediting its approval processes nowadays but there was a time when they knew their duty and did their due diligence. It's because of FDA regulations that the US mostly avoided the horrors of Thalidomide