Of Beer and Diapers

Urban legend has it that a large supermarket chain "did an analysis of customers' buying habits and found a statistically significant correlation between purchases of beer and purchases of nappies (diapers in the US). "

It was theorized that the reason for this was that fathers were stopping off at Wal-Mart to buy nappies for their babies, and since they could no longer go down to the pub as often, would buy beer as well. As a result of this finding, the supermarket chain is alleged to have the nappies next to the beer, resulting in increased sales of both.

This urban legend has grown to become one of the most famous exemplar of the usage of data mining. (Anyway, correlation does not imply causality!)