1. ‘Garbage In, Garbage Out’ — 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.
2. Garbage In, Gospel Out: this more recent expansion is a sardonic comment on the tendency human beings have to put excessive trust in ‘computerized’ data.