Quite frequently when you sit on or in a vehicle, the game seems to confuse the location and animation that should be used.

One example is watching someone drive a quad while laying on it in a prone position, another is sitting in an armored vehicle and your character looks like he is sitting on the roof with his legs in the vehicle.

This happens very frequently, but its random when it happens, and it is dependent on client as well. For example, I may see somebody as laying prone on the quad, but on their view they are sitting upright, and others may or may not see the glitched animation.