The First Computer Bug Was a Literal Bug

On September 9, 1947, Grace Hopper's team found a moth stuck in relay 70 of the Harvard Mark II computer — causing a malfunction. They taped it into the logbook with the note "First actual case of bug being found." The terms "bug" and "debugging" were already in use before this, but the incident made the metaphor literal and unforgettable.

Source: Naval History and Heritage Command — original logbook