We spent two hours yesterday debugging some code that kept firing early. It wasn't clear to anyone, including the people who wrote it, why this happened. We patched it with the C# equivalent of duck tape, but really, it still doesn't work right.
This incident shows how important it is to know what your code is supposed to do, and not to accept the code if it doesn't. Many tools exist to help—most notably, unit-testing tools like NUnit—but they have trouble with the specific problem that we encountered: events fired from black-box controls.
I will have more to say about this later.