I think we have all written some code that looks bulletproof to us. We run a set of tests with all the inputs we can think of, and it passes with flying colors (after several iterations of course).
Then we give it to someone else and it fails on their first or second attempt. They simply tried to use it in a way that we did not anticipate. It doesn't mean that we are dumb for not thinking of those possibilities; it just means that we did not think of every one of them.
>that looks bulletproof to us
And it pretty much is most times. For us. But indeed I did run into the "why would they use it like that though" scenario, where it fails. So I have to patch that usecase. At which point I go all "ok now it's really done" until another fringe usecase pops up and so on and so forth. And I did think about how sure I was it was bulletproof, humbling moment.