I've noticed in myself and other programmers an unconscious assumption that serious errors have serious symptoms. E.g. we must have fixed all the serious defect

I've noticed in myself and other programmers an unconscious assumption that serious errors have serious symptoms. E.g. we must have fixed all the serious defects by now, because there's nothing obviously wrong with the code. I've seen this proved wrong time and time again. Have you ever been maintaining some shipped code, and found a crash bug, where you're like: "I can't believe this shipped and we haven't heard about it." So, I'm probably treating this out-of-context comment more seriously than it deserves, but I like to emphasize the point. Even though there is a set of errors that are detectable from the shape of the source code, there is a much larger set of serious errors that are not.