Chocolate, coffee or tea - Contain caffeine, theobromine, or theophylline, which can be toxic and affect the heart and nervous system
Onions and garlic - Contain sulfoxides and disulfides, which can damage red blood cells and cause Heinz body anemia.
Grapes and raisins - Contain an unknown toxin, which can damage the kidneys
Macadamia nuts - Contain an unknown toxin, which can affect the digestive and nervous systems and muscle.
Potato, rhubarb, and tomato - Contain oxalates, which can affect the digestive, nervous, and urinary systems
Raw eggs - Contain an enzyme called avidin, which decreases the absorption of biotin (a B vitamin). This can lead to skin and hair coat problems. Raw eggs may also contain Salmonella.
Human tuna and/or raw fish - Can result in a thiamine (a B vitamin) deficiency leading to loss of appetite, seizures, and in severe cases, death. More common if fed regularly.
Also, absolutely DO NOT EVER give a cat Tylenol, as it will be fatal. Cat's livers cannot metabolize acetaminophen. Cats also can not metabolize aspirin or any other NSAIDs (non-steroidal anti-inflammatories) such as Advil or Aleve. Any preparations containing them, such as Pepto Bismol or Kao-Pectate (which contain acetylsalicylic acid, the active ingredient in aspirin), must be avoided.
****DO NOT EVER BRING ANY LILIES INTO YOUR HOME WITH CATS***
All parts of ALL lily plants WILL cause renal failure if injested by a cat. Even just licking lily pollen off the fur can cause symptoms of renal failure.