Cancelling "GET IN" Action into a MH-9 or AH-9 (Pilot or Copilot) locks character in seating position (Inside Heli Pose) outside the vehicle preventing further movement.
The Only way for the Player to escape the "Movement Lock" is to try to Get into the vehicle again.
If for example given vehicle is not there anymore the only way to regain control is to kill yourself and respawn.
{F19716}
While Getting in MH-9 or AH-9 (Pilot or Copilot) Cancel the action by Scrolling and Selecting "Cancel Action".
Similar issue was reported and might be related http://feedback.arma3.com/view.php?id=6669
In my opinion It has to do with the "CANCEL ACTION" Issue also, since it happens when someone gets into the Vehicle before them and their "GET IN" Action gets cancelled.
I confirm this in the lastest Dev build, not happening with KA-60, and you can fire your weapon and do some things, you would fix this when you get inside another vehicle nearby