During a cutscene in the mission Messing with the Man, a pedestrian's head can be seen laying on the floor. The mission is given by Mitch Baker at the Greasy Chopper. There is a slight possibility that it could be related to gang brutality, Mitch being a killer, or it's just a mechanic model. It could also be a pun from the name of the Greasy Chopper, because the head was chopped off. The same pedestrian can also be seen briefly during the mission Hog Tied, walking behind Tommy and Mitch.
The actual reason for the pedestrian's head in the ground is rather simple. The man in the ground uses the skin name
MSERVE, indicating that he is a waiter. During the Messing with the Man cutscene, he actually has a unique set of animations were he holds up a dish and walks around seductively. Because of these animations, it was likely that Rockstar intended for the man to be a female waitress, and thus, they quickly changed his Z coordinates to put him in the ground to cover for the mistake.