When a bug happens, it bugs us. However, a bug, that is, a little insect, does not "happens" it grows, procreates, and dies. Today we are not going to talk about ladybugs or mites, but about computer bugs. According to Techopedia, a bug refers to an error, fault, or flaw in any computer program or a hardware system. A bug produces unexpected results or causes a system to behave unexpectedly. We already know that both (the animal and the error) can irritate us, but there is a reason behind the computer bug be called bug? The answer is yes. The image below shows the nosey responsible for this word:  

source: National Museum of American History

"On September 9th, 1947, an error in the US Naval-operated Mark II computer was caused by a moth being trapped between two electrical relays. William Burke, the operator who found it, was so amused that he took the moth and placed it in a log-book with the annotation “First actual case of bug being found”. This was meant as a pun and is certainly not the first time the word was used to denote errors. The log-book now rests on display in the National Museum of American History of the Smithsonian Institute, with moth still attached." (TECHOPEDIA)

    Despite having good evidence, as well as plenteous words of the Proto-Indo-European language family, it is not clear to trace its first usage. According to the National Museum of American History, Thomas Edison, an American inventor, talked about bugs in electrical circuits in 1870. In other words, it is not possible to assume the reason behind the title software bug, even knowing the more likely field to create it: technology. But it is possible to see that even having nothing to do with it (bug x computer bug) the meaning may change and is particularly likely to change in a field undergoing rapid expansion and development, such as computer technology (ALGEO, 2009, p. 209).

SOURCE

ALGEO, John. Words and Meanings. In: THE ORIGINS and Development of the English Language. 6. ed. Boston: Wadsworth CENGAGE Learning, 2009. cap. 9, p. 206-223. ISBN 978-1-4282-3145-0.