Many "human" foods are incredibly healthy for your dogs but there are a few that can be toxic to dogs.
Skip anything containing alcohol. Sometimes there are small amounts in sauces and gravies which you should avoid.
If peeled and pitted the flesh itself can be safe in moderation. Avocado trees contain a fungicidal called persin which is concentrated in the leaves, peel and pit but there are low levels in the flesh as well. The flesh is high in calories and fats so consuming larger quantities can lead to vomiting and diarrhea in some dogs.
It contains the stimulant theobromine which is highly toxic to dogs. Dogs process theobromine and caffeine slowly which leads to the toxic compounds to build up in their system.
They can splinter which can cause damage to the mouth, throat or intestines.
Caffeine is poisonous to dogs, it raises blood pressure and causes cardiac arrhythmias. Can cause tremors, seizures and cause your dog to lose muscle control. It also affects the gastrointestinal tract and can cause vomiting and diarrhea.
Corn on the cob
Husks can become lodged in the dog's small intestine causing a blockage or perforation as they cannot be digested.
Fatty leftover meat
It can cause pancreatitis!
Grapes & raisins
Can cause kidney failure and death, even in small amounts. The toxic substance is unknown but dogs cannot metabolize tannins, flavonoids and monosaccharides from grapes.
They can cause weakness, vomiting, affect the nervous system and even depression. Nobody knows why but it is best to skip them! Save them for yourself.
Onions, leeks & other alliums
Onions, leeks, and other alliums can cause gastrointestinal problems and red blood cell damage but a little garlic from time to time can be beneficial.
Xylitol is a sugar alcohol that is highly toxic to dogs. Check labels as some brands of peanut butter do contain it.
Ripe, red tomatoes are fine from time to time but unripe green tomatoes, the leaves and stems are potentially harmful.