This is one reason extremely fat passengers are seated to the front of the plane.  Moving the expected center of gravity forward is not very dangerous, but moving it back can cause the plane to become uncontrollable in normal flight (I was working at an airport where they told me about a PC-12 that had a very fat passenger sat in the back, they tried to take off but as soon as they got lift they realized the cg was behind the center of lift, basically the plane was uncontrollable.  It kept nosing up into a stall, wheeled over and nosed down through a hangar, no survivors.  This apparently happened in the 90s or something.  I was in Kansas)