On September, 9 1945 the scientists of the Harvard University while testing the computer Mark II Aiken Relay Calculator have found a moth which has got stuck between the contacts of the electromechanical relay. The work they performed required some description, and the word has been found – «debugging» (literally: disposal of an insect) – and nowadays it is used to describe the process of identifying and eliminating bugs which cause a computer to malfunction.
The removed insect was pasted into the computer log with the entry: “First actual case of bug being found”, and was then transferred to the computer museum.
More Details...
All details taken directly from provider content at http://blog.jetbrains.com/youtrack/2010/09/happy-testers-day