This question on Travel.SE recounts a situation where a passenger, about to disembark from a commercial airline flight, was asked by a flight attendant to wait for other passengers to "catch up".
This answer suggests that if too many passengers were to disembark from the front, while passengers at the back remained at the back, the plane might tip backward; and that the flight attendant was acting to avoid this possibility.
Is this really a risk on modern airliners, or even regional jets or turboprops? (The aircraft in the question at hand was a 737.) Are flight attendants trained to redirect or hold passengers in order to avoid it?
I saw http://aviation.stackexchange.com/questions/1659/is-passenger-weight-distribution-important-on-a-commercial-flight?rq=1Is passenger weight distribution important on a commercial flight? but it is about the aerodynamic effects of passenger distribution, which are not relevant when the airplane is parked on the ground.