Chocolate is toxic to both cats and dogs, although cats are smarter at refusing it! The toxic agent in Chocolate is theobromine. It is in all chocolate including white, but the worst is dark, and unsweetened baking chocolate.
Onions in all its forms can breakdown the red blood cells and cause anemia.
Grapes and raisins can cause kidney failure in both dogs and cats.
Milk and other dairy products except for kittens will cause diarrohea and other digestive problems in adult dogs and cats.
Yeast Dough. Before it is baked bread dough needs to rise and that is exactly what it would do in your dogs or cats stomach if eaten. It will swell and stretch the abdomen causing severe pain. In the process of fermentation it will also produce alcohol causing alcohol poinsoning.
Alcohol has the same effect on a dogs or cats liver and brain as it has on a human but it takes a lot less to do its damage. 2 teaspoons whisky given to a 5lb cat could cause coma. A dog will feel symptoms like vomiting, diarrohea etc
Sugar, fat, raw eggs, caffeine are also foods to keep away from dogs and cats. For example, an enzyme in raw eggs will interfere with the absorption of a specific Vitamin B.
Peach/plum/Apricot pips for dogs not only once swallowed can cause obstruction but the pip contains cyanide which is both poinsonous to dogs and humans. The difference is humans know how to eat them. Dogs don't.