Although the act of patting the top of your head while simultaneously rubbing your belly is a monumental feat of brain power and coordination, it's also extremely dangerous. Because performing this trick demands such a high level of focus and concentration, there's a very real risk that once you start doing it, you can never stop. Ever.
On an annual basis, there are more deaths attributed to head-pat/belly-rub exhaustion than to all household accidents combined. Corpses often continue the maneuver for up to three hours post mortem.