The most dangerous are small splintery ones like pork chop, fish and poultry bones which can puncture an intestine, causing an agonizing, expensive and life-threatening crisis. When you put bones in the garbage, add a slop of ammonia. This cuts the eventual smell as they decay, and may save a garbage hound from pain and death. Small bones from poultry or fish may be pressure-cooked until soft, ground up and added to dogs' diets, however. It's good nutrition for them.
Egg whites can cause a serious vitamin deficiency in cats, as does a diet of nothing but tuna.
Large amounts of onion, cooked or raw, can cause hemolytic anemia in which the dog's red blood cells are destroyed. Too much garlic can also cause a similar problem, although small amounts are good for dogs and they love it. Don't go overboard with chicken mush at one sitting for such dogs, since it contains a lot of calcium. If a small one eats a whole package of good chocolate, you're in deep doggie doo. If in doubt, call your vet, who can fix it as long as you get there promptly.