My husband and I have a small web design business and also do computer repair out of our home.  

One day a client brought back a computer we had sold him only a month earlier, saying it was over-heating and shutting down. 

My husband took the machine into the basement to check it out. Minutes later he came running wildly back up the stairs with the computer, threw open the back door, and ran outside.  He was screaming.

The machine was filled with thousands of cockroaches.

Apparently, the client lived in a cold apartment, and the relatively warm innards of his compute were a cozy home for bugs.

Source: Clients from Hell