This issue seems to be occurring on a random basis on a serverside level.
In any large scale multiplayer mission, there is a chance that if a player enters the copilot seat while a helicopter already has a pilot, the player will desync outside and experience some sort of PHYSX problem. Everyone else in the server can see the player 'bouncing' around the rotors of the helicopter, whereas the copilot who caused the problem cannot. The problem lies with the 'get-in' animation when large quanities of players enter a vehicle.
The MROT (Main rotor) is instantly disabled by the player colliding with the rotors. Any attempts to repair the helicopter, and even when the fault copilot disembarks from the vehicle, the MROT re-disables itself. A very strange, and spontaneous bug.
PLEASE FIX THIS! IT IS BREAKING THE EXPERIENCE OF SOME GAMES!