It's always scary when your dog eats something they aren't supposed to, but the good new is most toxic foods have to be ingested relative to the dog's weight to be deadly. A handful of Hershey Kisses might not be as big of a concern to a giant Great Dane than to a little Pug. However, there are some foods where even the smallest amount can really harm or kill a dog so it's important to know these as an owner.
This is the artificial sweetener found in products like gum, toothpaste, and soda. An amount as small as .1 grams can cause serious issues in a large dog. If you haven't already, quickly check that your dog does not have easy access to anything with xylitol in it, or any sugar alcohols for that matter.
Never, ever, ever, give your dog your leftover steak bone! Choking is always a concern with this. Cooked bones break and splinter differently than raw bones. They can internally scratch or pierce the digestive system as they make their way through the body. Some raw bones are okay, but they shouldn't be machine cut (since they can be sharp), or weight-bearing bones (since these are too hard and can break teeth).
Grapes and raisins
The general rule is that dogs usually have to eat a specific ratio relative to their weight when it comes to grapes and raisins. But, the toxic components in grapes is still somewhat unknown, so there have been reported cases of dogs dying from eating even a small handful of raisins. For this reason, grapes and raisins should be completely avoided since we don't know what triggers death in some dogs.
The exact ingredient that causes toxicity to dogs in chocolate is theobromine. Baking and dark chocolate have the most concentrated amounts of theobromine so do not let your dog near these. Milk chocolate has less levels of the toxic ingredient so it's not as big of a concern as dark or baking chocolate. White chocolate has almost none, so it is somewhat safe for dogs to eat.
CaffeinE and Alcohol
I put these two in the same category because it's (hopefully) common sense not to let dogs ingest either of these substances. These are not even that great for humans, so they are definitely not good for dogs (again, common sense). Caffeine is bad for the central nervous system, and alcohol causes breathing and the heart rate to slow down.
Onions and garlic
Always check foods like chicken broth since these are usually prescribed for ill or picky dogs. Many broths contain onion or garlic ingredients in some form. These are both toxic raw or cooked, so make sure you don't give any human food leftovers that contain onions or garlic.
If your dogs eats any of these foods, it's always a safe bet to call your veterinarian. Watch your dog very closely, if they have any changes in behavior or start to vomit or have diarrhea, take them in to a vet ASAP!