Although dogs are omnivores, garlic, onions, chocolate and grapes are highly toxic to them. Fruit seeds and pits in general should be avoided; those that are not toxic often present choking hazards to canines. Nuts, mold and alcohol are very dangerous to dogs as well.
While avocados cause only mild gastric upset in dogs, the skin, leaves, seed and bark of the fruit are much more problematic, containing a toxic chemical known as persin. Small amounts of flesh and oil are sometimes included in commercial dog foods and should not present a problem.
Raw bread dough or anything containing yeast should never be fed to a dog. The warm, moist conditions of the stomach create a breeding ground for live yeast cultures that cause the raw dough to expand in the abdomen. In worst-case scenarios, this can lead to the death of tissue. Furthermore, as the yeast multiplies, it can create alcohols that cause the dog to become inebriated.
Xylitol is a common artificial sweetener found in gum and some baked products. While it has no discernible affect on humans, in dogs xylitol leads to a rapid and severe drop in blood sugar levels, sometimes resulting in seizures and liver failure in as little as 30 minutes after ingestion.