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?"