Reminds me of the time I had written a physical simulation engine back in grad school and there was a "minus" sign error. Of course, the error was rare enough that we didn't notice it until after the code was used in a real production environment. Tracking down one minus sign in several hundred thousands of lines is a pain. Not to mention the uneasy feeling you get after you solve it, "How was everything ever working correctly before!? What else did we overlook?"
If I have to venture a guess, I guess you didn't have a comprehensive set of tests at the function/method level of the code? Having that would probably have caught the bug, because you would have written a test for correctly executing the code in that branch.
You're right. But it was after that pain-staking experience that I became fully engrossed in using unittests for all non-trivial functionality. Live and learn.
