[FabHK]

Or the well known phenomenon that when people are asked how many lifetime opposite-sex sex partners they've had, the average reported by men is about twice the average reported by women (though, by construction, they ought to be the same - just imagine a bipartite graph (undirected!) and count the lines going from left to right and compare with the number of lines going from right to left).

See eg. David Spiegelhalter, Sex by Numbers: What Statistics Can Tell Us About Sexual Behaviour, chapter 3.