On September 9, 1947 engineer William B. Burke and colleagues, testing Aiken’s Harvard Mark II Relay Calculator, found that a large dead moth, trapped between points at Relay #70, Panel F, caused the relay to fail. Familar with the use of the term "bug" in the engineering context, they removed the actual bug and entered the dead insect into a log book with the note, "First actual case of bug being found." ”This was first use of the term “bug” within the context of computing, and also perhaps the origin of the concept of “debugging” within the context of computing.
Traditionally Grace Hopper has been credited with this discovery. Although she was fond of retelling the story, Hopper was not actually the person who found the insect.