This is only if key bind for player forward movement and vehicle acceleration are the same key (W by default if I'm not mistaken)
Turn off vehicle. Hit key to get out (X by default). Move away from vehicle as fast as you can (W by default).
If player does this at any time during the get out animation, the vehicle registers this as acceleration and starts back up. Even when player waits till they think animation is complete it can still happen, but less likely.