1. Dairy Products
Cats can actually become lactose intolerant when they get older. If ingested, these cats can experience vomiting and diarrhea.
2. Fat Trimmings, Raw Meat, Eggs, Fish
Fat trimmings can cause pancreatitis in cats. For ram meat, eggs, and fish, there is a risk of vomiting, diarrhea, Salmonella or E. coli. Its safer to stay away from these foods.
3. Grapes and Raisins
Cats most likely won't try to eat grapes or raisins, but they still shouldn't. Dogs can suffer acute kidney failure from eating grapes or raisins so just to be safe, keep your cat away as well.
When made into cat food, tuna is perfectly fine for cats. However, tuna that is sold for human consumption may cause digestive problems for your cat. It can even cause a painful condition called steatitis, or inflammation of body fat, when fed to cats.
5. Onions and Garlic
If enough onions are eaten by your cat, it can cause problems. However, only a small amount given occasionally isn't too risky. Eating a clove of garlic or a green onion can cause digestive problems. Eating onions on a regular basis can cause anemia.
The main component of chocolate that makes it unsafe is something that's called theobromine. It's in all forms of chocolate (milk, dark, baking etc.). Consuming chocolate can cause heart arrhythmias, muscle tremors, or even seizures. Chocolate also contains caffeine which is unsafe for cats as well.
This is an artificial sweetener used in a lot of sugar-free foods including chewing gum. There is no record of cats getting sick from this product, but in dogs it can cause a severe drop in blood sugar followed by liver failure. It's better to be safe than sorry with this product.