Team A's receiver is running along team B's goal line. He goes airborne to catch a pass, and has a firm grip and control of the ball in the air while the ball is in the end zone. Before he can get two feet down, a Team B defender collides with him, sending him back over the goal line, and the ball is outside the end zone (between the one and the goal) when the receiver's second foot (or knee) hit the ground. He maintains control of the ball throughout and after the process of the catch.
So, control was established in the end zone, but the catch was not "complete" until the player was out of the end zone again, but the player maintained control throughout. Is it a TD, or completed catch inside the one?