On September 9, 1947, Navy Rear Adm. Grace Hopper (at the time just a lieutenant) was helping Harvard University computer scientists with their Mark II computer. The thing was malfunctioning, and finally someone discovered a moth stuck in a relay.
Hopper remarked that they were "debugging" the system.
Hopper's phrase is now universally used in computing.
The remains of the moth can be found in the Harvard group's log book at the Smithsonian Institution's National Museum of American History in Washington, D.C.
Hopper invented the first working compiler in 1952 ("Nobody believed that. I had a running compiler and nobody would touch it. They told me computers could only do arithmetic.") and co-created the COBOL language in 1959. (It was Hopper who insisted on a computer language in ordinery English, not machine-code.)