I once lost most of an afternoon debugging an issue where orders in a PHP e-commerce system would very occasionally fail. Turns out several months before, some

I once lost most of an afternoon debugging an issue where orders in a PHP e-commerce system would very occasionally fail. Turns out several months before, someone was doing some refactoring, moved some methods around, but also changed a "==" to a "===" in the process. Generally a good idea, but it slipped through to production without anyone noticing or breaking any tests. The issue ended up being that a rare code path in a tangentially related method would cause that method to return a float instead of an int. This propagated through, eventually causing a check of 0.0 === 0 to fail where previously 0.0 == 0 passed.