After Riggs shoots the valve on the napalm tank being worn by the criminal with the flamethrower at the start of the film, the legendary "Wilhelm" Scream can be heard as the criminal goes flying.

As Mel Gibson, Danny Glover and Chris Rock leave the Chinese restaurant (down the stairs), you can see the umbrella handed to Chris Rock hit him in the face.

In one of the takes when Riggs and Murtaugh crashed through the window of the office building, one of the window washers was actually hit (from the "making of's"). Obviously the one in the movie was not this take.