While all dogs drink milk as puppies, the milk we buy and drink is very different. Also, much like humans, some dogs can suffer from lactose intolerance. The consumption of milk could lead to vomiting and diarrhea, and while these are not life-threatening, they can contribute to serious bacterial exposure that is unpleasant for both you and the dog.
Chocolate:
Chocolate is well known for being bad for dogs; it contains caffeine and theobromine which are dangerous for many animals, including dogs. If eaten by a dog, chocolate can cause vomiting, dehydration, abdominal pain, severe agitation and in extreme cases, death.
Onions:
While onions are not too friendly to humans, they can do a lot more than just give your dog watery eyes. They contain compounds that can be harmful to dogs and might damage their red blood cells. If a dog eats too much of onions, a blood transfusion might be necessary to give him some healthy blood back.