The first computer bug was an actual insect

In 1947, engineers at Harvard found a moth trapped in a relay of the Mark II computer, causing a malfunction. Grace Hopper's team taped it into the logbook with the note 'First actual case of bug being found.' Although 'bug' was already engineering slang for a fault, this literal insect cemented the term in computing forever.

Source: Naval Surface Warfare Center, Dahlgren Division Log Book, September 9, 1947 (U.S. National Archives)