Don't try this at home!
N.N.C.
The controllability in pitch of an aircraft is dependent on the distance between the centre of gravity and the effective hinge line of the elevators. This forms a lever arm and the longer it is the more stable the aircraft. If you move the c of g too far aft it can reduce the effective lever arm to such an extent that it passes the stick-free neutral point. This is the point beyond which it is no longer possible to trim the aircraft for stable "hands off " level flight. Any displacement in pitch will become divergent and loss of control and/or structural damage follows faster than pilot reaction time.
There have been a number of such accidents over the years which reinforce the need to check your load and balance every time...........