First, let me elaborate on rudder capabilities, before giving a more direct answer. We have to define full rudder as "maximum possible deflection" and not necessarily moving the rudder to its full potential extremities. Commercial airliners have rudder limiting systems that allow only a certain amount of deflection according to airspeed at the time. Full rudder at 4000 ft traveling at 230 knots will be different than full rudder at 41000 feet travelling at 450 knots. Airliner components are not made for this type of deflection and can be damaged. If opposite full rudder inputs were applied, it could potentially damage the tail fin or any other part. To completely separate the tail fin, it would take quite a bit of deflection repeatedly. Think of it this way, applying full rudder right followed by rull rudder left would be like taking your car and turning the wheel all the way around to the right and then to the left while travelling 60 MPH. Provided you had no obstacles to hit, you could survive this, but if you kept doing it, your car would probably flip or wreck. Airliners can withstand a full rudder input to one direction at any time. Note that I emphasize full opposite full inputs, because it's not uncommon to apply rudder right then left to balance out. The rudders useage is not so much for steering the aircraft but more for stabalizing and balancing along with the elevators and ailerons. Most likely this question refers to AA Flight 587, which lost its tailfin from a series of over enthusiastic rudder inputs from the first officer. The aircraft, an Airbus A300-600, was rated for a full rudder input to either direction, but had nothing in its manual about opposite inputs. Weaknesses in this particular aircrafts tail were later discovered and are considered a contributing factor in the crash. The turbulence and advanced roll training that pilots were put through by AA was scrutinized after this crash and has since been changed. Other than this incident, I've not heard of any tails flying off from rudder inputs.