American college football game. 2,200 people in a section of the crowd each hold a large card, dark on one side, light on the other. each of these people has an instruction sheet that tells them which way to hold the card for each of 14 'stunts' (as they appear to be called). See pic for an example of what this looks like (think b/w pixel art)

The prank was that 3 guys from one team used social engineering to figure out how this was done, and who had the instructions, then replaced the original instructions with 2,232 slightly altered copies.

Game gets played. First 11 stunts are as expected, but the 12th has the cards make up the logo of the opposing team (not the people with the cards), the 13th has the writing reversed, and the 14th says, big and bold, CALTECH