Candies, gums, mints, baked goods and chocolate containing the "sugar free" sweetener xylitol are especially poisonous, causing rapid hypoglycemia (low blood sugar) and liver failure in dogs and possibly other species (ferrets).
Chocolate is toxic to pets. Granted, a 50 pound dog would have to eat about 50 ounces of milk chocolate (but only 5 ounces of baking chocolate) for a toxic dose, but much smaller amounts can cause vomiting and diarrhea.
Signs of chocolate toxicity include tremors, nervousness, vomiting, diarrhea, increased heart rate, and in severe cases, seizures and death. If you suspect that your pet has eaten chocolate, consult your veterinarian.
Other candies, such as lollipops and those with plastic components, pose a danger if ingested. Lollipop sticks and plastic parts can cause intestinal obstruction and potentially rupture the intestines, which is a life-threatening emergency.