Bug

A fault in a computer system, usually associated with software. The term apparently stems from the early (pre-transistor) days of computing when machines used myriad valves and miles of wire. An apocryphal tale has it that one machine refused to work and, on examination of its innards, revealed a moth which had expired across some terminals thereby causing a short circuit. Once 'debugged' the machine worked perfectly - or so it is said.

These days the term is used indiscriminately to describe any situation in which a system behaves differently to expectations, and it is a generally accepted view that ALL commercially available software contains bugs - they just haven't discovered them all yet.