I've noticed that my unit tests are really useful to work out the bugs, but then they might get stale. Once something is tested and worked out, it's like they

I've noticed that my unit tests are really useful to work out the bugs, but then they might get stale. Once something is tested and worked out, it's like they become a little redundant. In software of course re-iterating the tests is maybe cost free, whereas in physical production 'unit tests' introduce maybe non-negligible costs so the 'advantage' in not having to repeat them possibly makes a difference.