I hate bugs that I can't fix because the cause is buried in a third party architecture (like WCF) and disappears faster than the symptoms occur. I call these things weasels, as in those little bastards that pop their heads out and make a nuisance and disappear under the ground before you have a chance to ask what the heck happened. (The other dictionary definition fits the bill, too: a sneaky, untrustworthy, or insincere person. http://www.merriam-webster.com/dictionary/weasel)

I once saw an app show up on my taskbar for a split second and it knocked whatever I was working on out of focus .. I didn't know what the heck it was, there was no way to trace it .. so I created a Windows app called "Weasel Monitor" that basically monitored all processes and made a bunch of noise (showed up on a log) if a process's lifetime was only a tiny split second. This worked for me, and the term weasel stuck.

Could've gone with mole but that word is severely over-used in software to mean different things. (Spelunking, anybody?)