Well, I've never had a problem with this in multiplayer, I could be wrong though, it might happen in multiplayer.
It also happens with vehicles if you're not around to stare at them during upgrade.
I also noticed something similar if I hop out of my tank, order another tank close by to pick me up, then zoom off in the new tank: The pilot that picked me up will try to get in my old tank, but I'll come back later, and he'll be standing on the ground inside the tank. Then, I'll bump into the tank, knocking it away, and then the pilot will take control of the tank.
