On April 10 at about 4 a.m. the shell of a car, painted to look exactly like an MIT Campus Police cruiser, was placed atop the Great Dome. The lights on the car were flashing.
The "car" was actually the painted body panels of a Chevrolet Cavalier attached to a wooden frame, complete with a mannequin of an officer behind the wheel. The driver had several empty cups of coffee and a bag of donuts. The car was removed by about 9 a.m. and taken to the MIT Museum.
Michael Whitson also has some photographs of the hack (the car and the dome, another view with people wondering what to do with it, and the fake officer) on his home page.