The first computer bug was an actual moth!
Here’s the first “computer bug” found in the Mark II Aiken Relay Calculator, via Hacker News:
The First “Computer Bug” Moth found trapped between points at Relay # 70, Panel F, of the Mark II Aiken Relay Calculator while it was being tested at Harvard University, 9 September 1947. The operators affixed the moth to the computer log, with the entry: “First actual case of bug being found”. (The term “debugging” already existed; thus, finding an actual bug was an amusing occurrence.) In 1988, the log, with the moth still taped by the entry, was in the Naval Surface Warfare Center Computer Museum at Dahlgren, Virginia, which erroneously dated it 9 September 1945. The Smithsonian Institute’s National Museum of American History and other sources have the correct date of 9 September 1947 (Object ID: 1994.0191.01). The Harvard Mark II computer was not complete until the summer of 1947. Removed caption read: Photo # NH 96566-KB First Computer “Bug”, 1945.