High-speed or 'weird' collisions between lots of vehicles at once or badly-made vehicles can cause errors.
I made a vehicle called the 'HyperJeep' for my server, once, which was quite fun until the game crashed because you accelerated to 1000x the normal wheel speed a jeep is supposed to go at. (Assuming you could actually catch the vehicle because it started bouncing around the room as soon as it spawned)