The first computer "bug" was a real bug.
In 1945, engineers found a moth in Panel F, Relay #70 of the Harvard University Mark II Aiken Relay Calculator (a primitive computer).
The computer was running a test of its multiplier and adder when the engineers noticed something was wrong.
The moth was trapped, removed and taped into the computer's logbook with the words: "first actual case of a bug being found." (see picture above)
Then there it begins, the operators said that they had "debugged" the machine.. thats the story of "debugging a computer" born...