A term in software programming used to describe a coding problem (or "bug") that seems to defy testing by presenting itself exclusively in published, released or demonstrated code, but not at any point during debugging. It disappears or alters its characteristics when an attempt is made to study it.
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).
Ben: Hey! After three weeks, I finally finished that interface you asked for. Here, try it out.
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!!!