Yes, even a few can be very dangerous.
Grapes and Raisins: Grapes and raisins can cause kidney failure in dogs. As little as a single serving of raisins can kill a dog.
Onions: Onions destroy red blood cells and can cause anemia.
Chocolate: Chocolate can cause seizures, coma and death. Baker’s chocolate is the most dangerous. A dog can consume milk chocolate and appear to be fine because it is not as concentrated, but it is still dangerous.
Coffee, Coffee grounds, tea and tea bags: Drinks/foods containing caffeine cause many of the same symptoms chocolate causes
Macadamia Nuts and Walnuts: Macadamia nuts can cause weakness, muscle tremor and paralysis. Limit all other nuts as they are not good for dogs in general, their high phosphorous content is said to possibly lead to bladder stones. Exception to this rule seems to be PEANUT BUTTER. However- always use Salt/Sugar free ORGANIC Peanut butter (sugar encourages cancer growth) free . Use only ORGANIC peanut butter as regular peanut butter has lots of toxins and is full of pesticides!
Animal fat and fried foods: Excessive fat can cause pancreatitis.
Bones: Cooked bones can splinter and damage a dog’s internal organs. Raw Bone should always be supervised as a piece can always break off and cause problems. Try frozen oxtails or frozen knuckle bones then take the bone away before the dog can swallow a final small piece whole. It's a good natural way to clean teeth too.
Tomatoes: Tomatoes can cause tremors and heart arrhythmias. Tomato plants and the most toxic, but tomatoes themselves are also unsafe.
Avocados: The fruit, pit and plant are all toxic. They can cause difficulty breathing and fluid accumulation in the chest, abdomen and heart.
Nutmeg: Nutmeg can cause tremors, seizures and death
Apples, Cherries, Peaches PITS and similar fruit are great for your dog - HOWEVER, the seeds of these fruits contain cyanide, which is poisonous to dogs as well as humans. Unlike humans, dogs do not know to stop eating at the core/pit and easily ingest them. It can also become lodged in the intestines and kill the dog in 24 hours with no warning.
Raw eggs: Raw eggs can cause salmonella poisoning in dogs. Dogs have a shorter digestive tract than humans and are not as likely to suffer from food poisoning, but it is still possible. BEST to use ORGANIC EGGS if you do raw. Scrambled lightly is best!
Salt: Excessive salt intake can cause kidney problems.
Mushrooms: Can be deadly- never let your pets chew on mushrooms found in your yard. Only safe "food" mushrooms are shitaki, maitake and reishi.
Xylitol: even a small amount can cause liver failure and death. Read more at here and in this article at About.com.
Sugar and Corn Syrups. EVEN ORGANIC IS BAD! (this does not include Honey or Molasses though they should only be in small amounts and never for cancer dogs)