The bugs that always disappear when I try to demonstrate them to others - my favorite. Reminds me of the "Tom Knight and the Lisp Machine" koan. This is largely true, but I remember a failing piece of authentication hardware that I didn't understand but was convinced was failing. Every time I called someone over it would work, so I couldn't get it replaced. Eventually the failure rate got so high that everyone agreed the damn thing had failed, "dead as a brick". But until then I was SoL. What are the chances that something 99% towards "dead as a brick" would always work around a superior.