When it doesn't add up

I bring you the High School Prom Theorem:

We suppose that on the day after the prom, each girl is asked to give the number of boys she danced with. These numbers are then added up giving a number G. The same information is then obtained from the boys, giving a number B.

Theorem: G=B

Proof: Both G and B are equal to C, the number of couples who danced together at the prom. Q.E.D.

One of the things it shows is that when, in surveys, men report having many more sexual partners than women, something is amiss. (Via.)