If a player can get on the roof of an armored ground vehicle, he can go into the lower substance of the Crouch stance (Crouch, then CTRL + S) and aim directly down. This will allow him to see the inside of the vehicle, and not only that but also shoot through the roof. He is therefore able to kill the crew with even something like a pistol!
Tested on:
Slammer UP (gunner in this vehicle is invisible)
Varsuk
Modded Vehicles (RHS Escalation) - BMP models, BMD, T-90, etc...
it seems to work on pretty much all vehicles.
Players can get on the roof in the vanilla base game by simply jumping off a balcony / using a helicopter insertion / paradropping... the possibilities are endless
video explaining the bug - https://www.youtube.com/watch?v=qQNmJNiLuhQ