Yet another bug has been uncovered in Battlefield 4, related to an invisible hit box that can block projectiles and prevent a kill being made. It was discovered by YouTuber Jack Frag, and there's a video to demonstrate.
It stems from reviving players with the defibrillator. In multiplayer there is a short window after death when a player can accept a revival. This bug makes it so that if a live player stands behind a player waiting for revival, it acts as a shield, blocking bullets and even tank shells. The video makes it a little more clear.
A DICE QA lead has now said that they'll be looking to fix the bug as soon as possible.