I hadn't heard the term heisenbug! Here's a link for anybody else who may not know the term (and others of that sort): http://en.wikipedia.org/wiki/Unusual_software_bug

Wikipedia's attempts to come up with names for these classes of things that "everyone" already groups together, but which have no widely used name, because it has to give the article some sort of title, are a pretty nice feature of the encyclopedia. "Unusual software bug" has a nicely euphemistic feel to it too, sort of like "anomalous condition".