From the Hacker's Dictionary: heisenbug - A bug that disappears or alters its behavior when one attempts to probe or isolate it. (This usage is not even particularly fanciful; the use of a debugger sometimes alters a program's operating environment significantly enough that buggy code, such as that which relies on the values of uninitialized memory, behaves quite differently.)
Heisenfeature - A feature that disappears or alters its behavior when one attempts to probe or isolate it. Also known as a human.
I write software. Sometimes it works as expected. Sometimes I work as expected, but you'll never know for certain. I'm female, geeky, am the proud mother of two smart daughters, and am married to the best husband ever.