Word of the day: Heisenbug.
Their possible presence leads to the Heisenbug uncertainty principle: it is never possible to be sure there are no more bugs :)
I am fighting a tenacious Heisenbug just now and wishing I could pause the universe temporarily to examine the server when an error occurs on the client. I wonder if the developers of The Matrix began this way?
P.S. Yes of course there is a Wikipedia entry for Heisenbugs, as well as Bohrbugs, Mandelbugs, and Schroedinbugs. And my favorite, the Stotle, “the incorrect output of a computer program that contains no bug”.