The bug both exists and doesn't exist and simultaneous points in time -- the reference is to Heisenberg's Uncertainty Principle (or to be direct, the quantum cat in the Heisenberg scenario).
Mike: Great! I'll just click on the "Save" button... Hey, the browser crashed!
Ben: What? Let me look at it... All my debug information looks fine... I'm not getting any errors at all!
Mike: It's all fucked up! Look! It doesn't even work! What are you trying to do to me?
Ben: B..but I just ran it on my own server and it works fine! Look, I'm not even getting any errors from your Save! It can't be a bug!
Mike: IT'S FUCKING BROKEN! You've disappointed me for the last time! YOU'RE FIRED!
Ben: CURSE YOU, HEISENBUG!!!