Sporting pranks

Sometime ago, I linked to this wonderful Economist essay about pranks. One of the examples in the essay is the following:

The best colleges strive to out-prank one another. Students at Yale scored a big victory during last year's football match against Harvard when they passed out pieces of paper to thousands of fans on the Harvard side of the stadium. The fans were told that, when held up, the bits would spell “Go Harvard”. In fact they spelled something else (see photo that opened this article).

I am not sure if this prank is in the same league as the one pulled off by Yale, but Bruce Schneier calls it "the cleverest social engineering attack I've read about in a long time."