A moth lodged in a relay of the Harvard Mark II, and the engineering team taped the insect into the machine’s logbook on September 9, 1947. The entry jokingly called it the “first actual case of bug being found.” The phrase was funny precisely because “bug” already described technical faults. This was an actual insect, not the origin of the word.
A machine stopped by something small
The Mark II was an electromechanical computer at Harvard. Relays opened and closed electrical contacts to carry out calculations. A physical obstruction could therefore interfere with operation. In the surviving logbook, the moth became both evidence of a fault and a joke written down by the people maintaining the machine.
The Smithsonian preserves the logbook as an artifact. The Computer History Museum dates the entry September 9, 1947 and says Grace Hopper was present and later helped make the story famous. Accounts sometimes name her as the discoverer. The logbook itself does not require us to assign sole credit for finding the moth to Hopper rather than the team at work.
Why “first” needs care
The expression “bug” for a mechanical or electrical problem was used long before 1947. Engineers had also spoken of debugging. The logbook’s “first actual case” was a playful distinction between a metaphorical problem and a literal moth. Describing it as the first computer error would be plainly wrong; machines and programs had failed before.
The incident survives because an ordinary maintenance action left a tangible record. The taped moth gives historians an object that can be examined, photographed and dated. It also shows how a working joke can become a foundational myth when later retellings omit the word “actual.”
A small artifact with a long life
Grace Hopper became a leading figure in computing, and her later lectures helped carry the anecdote to wider audiences. Her contributions deserve attention without making the story depend on an unproved claim that she personally found the insect. The artifact belongs to a team and a technological culture that was still inventing its vocabulary.
The image above is of the original logbook, not an AI rendering. Explore Science & Space or return to the September 9 edition.
Sources and method
The date and artifact are checked against the Smithsonian’s object record and the Computer History Museum’s dated entry. The latter distinguishes Hopper’s role in popularizing the story. The U.S. Navy archival image is identified and credited above.
Written and fact-checked by the Discover Today in History editorial team.
