The first computer bug was an actual bug

On 9 September 1947, engineers working on the Harvard Mark II computer found a moth trapped in one of the machine's relay switches, causing a malfunction. Grace Hopper's team taped it into the log book with the note 'First actual case of bug being found'. The word 'bug' to describe engineering faults predates the moth — Thomas Edison used it in 1878 — but the Harvard incident solidified its use in computing. The logbook page with the moth is preserved at the Smithsonian National Museum of American History.

Source: Smithsonian National Museum of American History archives