If you spam the ctrl key to raise/lower weapon while trying to board a vehicle it glitches the animation and does some strange things.

With a quadbike you sit down normally but then the animation glitches and makes you go prone inside the quad, then raises your stance so you're standing on top of it.

With a helicopter it just messes with the view so up is left and down is right (for me anyway, friend couldn't reproduce).

While it's glitched you still have control of the vehicle (if in driver seat) so you can drive around while walking on top of it, looks pretty funny really but I think the effect is only client side. Noticed this bug since the Alpha but only just decided to report now its released, hope I wasn't too late :x