The Mark II was one of the earliest computers built. Designed by IBM, it performed mathematical calculations that were to complex for human "computers" to do. The Mark II took up 4000 square feet of floor space, was given instructions in the form of punched cards and had a clock rate of 8Hz (for comparison, an iPhone 8 has a clock rate of 2390000000Hz).
The Mark II
On September 9, 1947, researchers at Harvard were running a computer program through the Mark II computer when they detected an error in its calculations. The program (i.e. the punched cards) was not at fault, so they set out checking all 13,000 relays. At 15:45, William “Bill” Burke found the problem. A moth was had been squashed in Relay #70, Panel F and had jammed the relay. He took it out (performing the first debug) and taped it in the notebook with the note "first actual case of bug being found."
The "Bug Report"
Contrary to popular belief, Grace Hopper was not the person that discovered the moth. She did love telling the story though, and even drew a picture of a Mark II infested with worms.
This also wasn't the first time the word "bug" was used: in 1878 Thomas Edison used the word in a letter to a friend and the word was commonly used by 1947. The moth incident did popularize the word though, and it is credited with coining the word debugging.
Special thanks to 
