It's like removing bounds checking (a runtime safety feature).

A manufacturing line is more analogous to software during use than it is to software during creation.

I think its kind of like when in Clojure, you only activate validation on the boundaries of your program in production, and disable the validation in the internals of the program.

Because a factory would still test its input, or the input would have been tested by the supplier.

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.