The moth was a hardware bug

At 3:45 p.m. on September 9, 1947, operators of Harvard’s Mark II calculator found a moth lodged in relay number 70 on panel F. They removed it, taped it into the machine’s logbook and wrote beneath it: “First actual case of bug being found.”

The sentence contains its own correction to the usual story. Nobody calls something the first actual bug unless “bug” already has a figurative meaning. Engineers had used the word for faults and defects long before 1947. The moth did not give computing a new term. It supplied an unusually literal joke.

It was not a software bug either. The Mark II performed calculations through thousands of electromechanical relays—physical switches whose contacts opened and closed. The insect interfered with one of those components. Whatever instructions the machine had been given, the moth had not made an error in them. It had become part of the hardware.

That distinction has blurred because the surviving page is such good evidence. Most malfunctions enter a maintenance log as descriptions: a bad contact, an incorrect result, a replaced part. Here the operators could attach the cause itself. For one entry, the logbook became a specimen case.

Grace Hopper, then working with the Mark II team, later made the story famous in lectures. She is often credited with finding the moth and writing the note, but the Smithsonian, which holds the logbook, says it was probably not hers. The Computer History Museum more cautiously credits the team.

The anecdote is remembered as an origin story for software language. Its appeal is almost the reverse. A word that had already become metaphorical briefly acquired wings, relay coordinates and a strip of tape.