On September 9, 1947, engineers working on Harvard's Mark II computer traced a problem to Relay #70, Panel F. The problem was a moth. They pulled it out, taped it into the logbook, and wrote: "First actual case of bug being found."
The story everyone tells
The popular version goes like this: Grace Hopper found a moth in a computer, and that's why we call software problems bugs. It's a great story. It's also not quite right.
What actually happened
Grace Hopper was a pioneering computer scientist and part of the team working on the Mark II, and she told this story often in later years, which is a big part of why it's famous. But the logbook probably wasn't hers, and nobody knows for sure who found the moth or wrote the note.
And the word "bug" was already old. Thomas Edison was complaining about "bugs" in his inventions in the 1870s. Engineers had used it for decades to mean a small, annoying fault. Read the note again: "First actual case of bug being found." The joke only works because everyone already called glitches bugs. The engineers were delighted to finally find a literal one.
Where the moth is now
The logbook, moth still attached, is at the Smithsonian's National Museum of American History. A dead insect from 1947 is one of the most famous artifacts in computing history, which is a pretty good career for a moth.
The point
The best tech stories get simpler every time they're retold, until the real details fall off. The real version here is better anyway: a team of engineers, a broken relay, and someone with a sense of humor and a roll of tape.