Garbage In, Garbage Out
(GIGO) /gi:'goh/ Wilf Hey's maxim expressing the fact that computers, unlike humans, will unquestioningly process nonsensical input data and produce nonsensical output. Of course a properly written program will reject input data that is obviously erroneous but such checking is not always easy to specify and is tedious to write.
GIGO is usually said in response to lusers who complain that a program didn't "do the right thing" when given imperfect input or otherwise mistreated in some way. Also commonly used to describe failures in human decision making due to faulty, incomplete, or imprecise data.