This is not a complete list and some cats tolerate more than others. Please comment if you think we forgot something 🙂

Alcohol – even 1 tablespoon can cause liver and brain damage

Chocolate – can cause heartproblems, tremor in muscles and seizures

Caffeine – can cause arrhythmia and tremor in muscles

Dairy products – some cats (and dogs) are lactose intolerant and dairy products can cause vomiting and diarrhea

Fatty food – van cause pancreatitis, diarrhea and vomiting

Grapes and raisins – the toxicity of grapes on cats has never been proven but it is not recommended to  feed cats on grapes or raisins

Onions and garlic – doesn’t affect cats in small quantities but big quantities can cause upset stomach and anemia

Xylitol – it hasn’t been proven to affect cats but it is not recommended to feed cats on xylitol

Dogfood – cats need more protein than dogs and therefore it is not suitable for cats, but small quantities doesn’t hurt the cat

Liver – a small piece is ok but a lot can cause vitamin A poisoning

Yeast – expands in the stomach and can cause upset stomach and gas. In large quantities the stomach can burst

Tuna – the type that is meant for human consumption can cause upset stomach and inflammations in the body. It is better to stick to tuna that is meant for cats

Resources:
https://www.vets-now.com/2017/02/foods-poisonous-to-cats/
https://pets.webmd.com/cats/ss/slideshow-foods-your-cat-should-never-eat
https://www.petmd.com/cat/emergency/poisoning-toxicity/e_ct_human_food_poisoning