I work in software testing, and I've seen this many times actually. Small bugs that I notice because I'm actually reading the code, which became inconsequential because that code path is never used anymore or the result is now discarded, or any of a number of things that change the execution environment of that piece of code.

If anything I'm wondering the same question about you. If you find it so inconceivable that a bug is hiding in working code that is held up because the calling environment around it, than you must not have worked with big or even moderately sized codebases at all.