I think this will answer your question:
Many dog treats and prepared foods contain liver so it may surprise you to find liver on the bad foods list. In small amounts liver is good for your dog, but if the liver intake is too high it can cause nutritional problems because liver has a high content of vitamin A. Consumption of this vitamin in large amounts can lead to vitamin A toxicity, or hypervitaminosis A. If your dog eats raw liver or consumes three servings of cooked liver a week it could lead to bone problems. Feeding liver to a dog taking Vitimin A supplements can lead to an overdose of Vitamin A.
Symptoms of hypervitaminosis A are deformed bones, excessive bone growth on the elbows and spine, weight loss and anorexia. If left unchecked, hypervitaminosis A has in some cases caused death.
http://mooreshaven.com/pets/dogs/saf...foodslist.html