things that make you go "hm"...

In the early 1980's, an I.R.S. research officer in Washington named John Szilagyi had seen enough random audits to know that some taxpayers were incorrectly claiming dependents for the sake of an exemption. Sometimes it was a genuine mistake (a divorced wife and husband making duplicate claims on their children), and sometimes the claims were comically fraudulent (Szilagyi recalls at least one dependent's name listed as Fluffy, who was quite obviously a pet rather than a child).

Szilagyi decided that the most efficient way to clean up this mess was to simply require taxpayers to list their children's Social Security numbers. [...] Szilagyi's idea was [put] into law for tax year 1986. When the returns started coming in the following April, Szilagyi recalls, he and his bosses were shocked: seven million dependents had suddenly vanished from the tax rolls, some incalculable combination of real pets and phantom children. Szilagyi's clever twist generated nearly $3 billion in revenues in a single year.

Source: New York Times Magazine article by Stephen J. Dubner and Steven D. Levitt, authors of Freakonomics; emphasis mine. ;)