A good example of being locked into a bogus design is old Humvee turn signals. I have never once had a turn signal on a car fill up with water like a little fishtank, but when PMCSing Humvees I would run into this from time to time. IIRC you had to drill a hole in the bottom to let the water out.
Another case is SINCGARS radios. I don't know if they still use them, but god help them if they do. You had to load crypto on them using arcane magic that almost nobody knew how to do, including me, and I took the 2 week class. If you didn't have the crypto set up properly you couldn't talk to people except on the emergency channel in the clear. I can't count how many times we weren't able to talk to each other because the radios were all screwed up, there was a lot of resorting to cell phones when they were available. You would think something as fundamental as radios would be figured out and fool proof, but in my experience off the shelf walkee talkees would have been far more effective, although not as secure in theory. In practice the mujahideen aren't intercepting and exploiting US communications, but they certainly benefited from the radios being dicked up half the time.