I think you mean contracts or defensive coding. If someone calls my code wrong they will have to think to test that particular case themselves, which is hard. Unless I've written contracts; then when it blows up they'll know what they did wrong.
The question we have to answer to properly understand what went wrong is where did the argument originate. If it's being generated inside function A that then calls function B with it, a test of A should fail when it calls B with the wrong argument. In any case, I would imagine the test coverage in the A-B system is lower than it should.
