Bots
A special case is that of a run-away bot, a program that was designed to be helpful by automating a drudgerous task, but due to poor programming or poorly understood circumstances, goes out of control and hammers a server unceasingly at high rate. A common case is a web spider which accesses too many pages on a web server too quickly at the expense of the server's intended audience.
Read more about this topic: Server Hog