The first documented computer bug was an actual moth stuck in a relay.
In September 1947, operators of the Harvard Mark II computer found a moth trapped inside one of its relays and taped it into the logbook with the note: first actual case of bug being found. The logbook page is now in the Smithsonian.
Why is this true?
Engineers had used the word bug for mechanical glitches since Thomas Edison's time, which is why the team found the moth so funny. Grace Hopper, who worked on the Mark II, helped make the story famous.
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More about inventions
Most inventions arrive by accident, in the wrong order, or decades before anyone finds a use for them. Microwaves, sticky notes, safety glass and vulcanised rubber all started as mistakes or side effects. The facts here trace how the thing actually came about, note who noticed the accident and why it mattered, and point at a source so you can follow the full story if it hooks you.