I was testing a radar system that controlled the guns and missile launchers on a naval combat ship. The radar system consisted of a Fore subsystem, Aft subsystem and a Weapons subsystem. After several months of testing we discovered that if we started the subsystems in a certain sequence the Fore guns and Aft guns would reverse and point at the ship. The scary thing was this bug had existed for years without being detected! Fortunately all ships have mechanical interlocks to guard against any weapon pointing at the ship itself and committing suicide.
Do you have a bug whose story you love to tell? Let me know!