GIGO is usually said in response to users who complain that a program did not "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.
2. Garbage In, Gospel Out. This more recent expansion is a sardonic comment on the tendency human beings have to put excessive trust in "computerised" data.
see also KIBO