Originally Posted by
Hammersmith
If the player takes possession of the ball outside the end zone, then enters the end zone and is tackled, then it's a safety(unless the ball was caught inside the 5 and the momentum of the catch takes the player into the end zone). But if possession begins inside the end zone, then the outcome is a touchback no matter how much the player moves around inside the end zone. I think the only way it might become a safety is if the player takes possession within the end zone, then steps out of bounds beyond the goal line. I'm not sure if that would be a safety or a touchback.
Think of it like an interception in the end zone(both are changes of possession inside the end zone). Player runs around, gets tackled. Touchback, not safety. I believe a safety requires possession to be established in the field of play, followed by entry into the end zone. The muffed punt did not establish possession, it only created a live ball situation.
But I'm probably wrong about all the philosophy(but not the application).
edit: started post before indy's; yep, wrong about philosophy, right about application