The ESRB wouldn't let Valve release the cinematic trailer for Left 4 Dead 2, so what did they do? They "leaked" it, obviously
Though perhaps that impression would have changed had I seen its cinematic trailer as soon as Valve had wanted to release it. The problem was, the Entertainment Software Ratings Board weren't so hot on the game, including its marketing materials, so that left Valve with one option: leaking the trailer themselves.
