6. If The Player Shoots The Moon, It Will Change Size (3D Universe Only)
The first game to feature the moon was GTA III, and it was a mainstay of the 3D Universe from that point on, with one of the more amusing easter eggs being that you could shoot the moon with a sniper rifle and cause it to change size instantaneously, getting bigger with each shot before returning to its smallest size. It serves no narrative or gameplay function whatsoever, but is a pretty imaginative hidden feature nonetheless, and one that's incredibly easy to miss unless you're just screwing around or do it by pure accident.