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.