All bodybuilders have looked in the mirror during a workout and noticed that they were getting a terrific pump in a particular muscle group. Usually this is encouraging, so you continue to train that bodypart.
But after a certain point, you notice that the pump is suddenly gone. Then you should remember the last set on which you still retained the pump. That's the set you should have quit on.
If you do too many sets, the body goes into shock. It does that to protect itself from injury. If you could continue to pump a muscle, you'd rupture capillaries and injure yourself. That's why your pump goes down. In other words, the loss of pump (over-tonus) is caused by overtraining.
Over-tonus also occurs in cases of generalized overtraining, when the muscles begin to slowly shrink, regardless of how much you train them (or probably because of how much you train them).
I think that anything over 8-10 total sets per bodypart will eventually lead to overtraining and overtonus.