Technicians working on the Harvard Mark II computer discovered a moth trapped in one of its electromechanical relays, causing a malfunction. They removed the insect, taped it into the operations logbook with the notation 'first actual case of bug being found,' cementing a term already used loosely for glitches into permanent computing lore. The logbook page survives today at the Smithsonian.
The Mark II, built for the U.S. Navy, was a room-sized behemoth of relays and wiring, primitive by later standards but revolutionary for its era. Its operators, working under the eventual supervision of Grace Hopper — who would go on to help develop COBOL and become one of computing's most celebrated pioneers — kept a meticulous logbook of every test run and every failure.
On this day, technicians traced a persistent error to Relay #70, Panel F, where they found the culprit: a moth had flown into the mechanism and been crushed between the contacts, physically jamming the switch. Someone taped the moth's remains directly into the log next to the entry, writing the now-famous phrase about the 'first actual case of bug being found,' a wry nod to engineers' pre-existing slang for technical faults, a term that dated back at least to Thomas Edison.
The logbook page, moth and all, eventually made its way into the collection of the Smithsonian National Museum of American History, transforming a minor maintenance annoyance into one of computing's most cherished artifacts — a literal bug behind the metaphor that every programmer still uses when software misbehaves.
Key people: Grace Hopper