EN48 asks the right question:
Let's cut through the cheese. Jack stall or servo stall is a design shortfall, nobody wants it to happen. Accidents have been caused by jack stall, and it is yet another way for the pilot to be blamed for a marginal aircraft.
To my knowledge, only Eurocopter aircraft get jack stall, probably because their designs have been grown so much, and their designers probably decided to accept jack stall by staying with more marginal servos. There is no doubt that the aircraft can be safely flown, but the pilot must be more aware of maneuvers, weights and altitudes.
Jack stall is due to having servos that are too weak, and no modern helicopter would be passed by the US military with this "feature". In fact, most dual servo helis are designed so that ONE servo can resist the maximum blade forces, so even with a hydraulic system out, you can't get jack stall. The prime cause of jack stall is the growth of the mass of the machine without re-fitting the servos, so the original servos are asked to work harder as the all up mass is increased and the rotor comes closer to stall in normal maneuvers. It is cheaper to leave the same servos, and also lighter, so there is a strong rationale to let the aircraft come closer to jack stall with each MGW increase.
Don't be fooled by apologists, at high mass and high altitude, it takes very little to cause jack stall. The real name for jack stall could be "loss of control" as discussed by a few tales on this thread. When the servos are back-driven, the pilot is a passenger, and a crash is more likely to occur.