Civilian Lemmings
Go to the gym in Las Venturas, and through the bomb garage next to it. Stand in that alley and look at the building across the street to your right; and you'll see people jumping and walking off the roof. It will only happen if you stand in that alley and watch it from there. It's pretty much a Lemmings Easter Egg since Rockstar North made Lemmings (the company was known as DMA then)