Wisdom by Giles
"...this illustrates a fundamental rule. Never trust business users to analyze their problem space correctly, whether they irritate you or not. If you build a system to their inaccurate understanding of the data, they'll blame you. (Conversely, if your code exposes flaws in their business process, they'll tell you "we can live with the way you handled that.") Their job is to give you the data and tell you the results they want. Your job is to give them the results - and since interpreting the data happens after they give you the data and before you give them the results, that means figuring out the data is your concern."
-- Giles Bowkett