Often it is tempting to share our food with the family pet, and most of us are guilty of slipping the odd tasty morsel. If you do indulge your cat, it is strongly advised you check out this list which contains some common foods which are either poisonous or have the potential to be dangerous to your cat. This list contains common foods, but is by no means extensive. As a rule of thumb, if in doubt, don't give it to your cat. It may save him a great deal of pain and suffering and you an expensive veterinarian bill.
I would like to hope that nobody would actually try giving a cat alcohol. Alcohol can cause intoxication, gastrointestinal irritation, respiratory distress, coma and death.
Apricot, cherry, peach pits, apple seeds:
All of the above contain cyanogenic glycosides which will result in cyanide poisoning. This interferes with the ability of the blood to release oxygen into the tissues, so the cat effectively suffocates while it's blood is full of oxygen. 
Avocado contains a toxin known as persin. It is found in all parts of the avocado and is toxic to most domestic animals.
The symptoms include gastrointestinal irritation, vomiting, diarrhea, respiratory distress, congestion, fluid accumulation around the tissues of the heart and even death. 
May cause vomiting in cats.
Cooked bones can splinter and cause an obstruction in the gastrointestinal tract, therefore they should be avoided.
Chocolate, Coffee and Tea:
Chocolate contains naturally occurring methylxanthines caffeine and theobromine. The concentration of theobromine is much higher than that of caffeine, but both cause toxicity in cats (and dogs). These chemicals act as a diuretic, causing the body to lose fluids, act as a cardiac stimulant, causing it to race and is also a central nervous system stimulant.
All types of chocolate should be avoided, including dark, milk, white and cocoa powders.
Grapes and raisins:
These have been known to cause kidney damage. The active ingredient hasn't been identified.
Macadamia nuts have been known to cause tremors and muscle weakness in dogs, therefore it's best to apply caution and avoid giving them to cats.
While this won't kill your cat, some cats are lactose intolerant and drinking cow's milk will cause a tummy upset.
Weaned cats no longer need to drink milk, however if you do want to give your cat some milk as an occasional treat them the best solution is to buy some cat milk from your supermarket.
There are many types of mushrooms which are poisonous. While dogs are far more likely to eat mushrooms than cats, it is advised you avoid giving your cats any type of mushrooms to be on the safe side.
While fat isn't toxic to cats per se, it's not healthy to routinely feed your cat a diet high in animal fats as it will lead to obesity. Large amounts of fat can also cause pancreatitis.
Small amounts of liver are fine to eat, but if fed too often it can result in Vitamin A toxicosis.
Onion and Garlic:
Onions contain sulfoxides and disulfides which damage red blood cells and can cause haemolytic anaemia.
Be careful when feeding your cat human foods, some people will recommend baby food for a cat who's lost it's appetite. This is fine, but make sure the food doesn't contain any onion, flakes, powder etc.
Onion causes greater problems than garlic.
Potato (green parts and eyes):
These contain solanine glycosides which can cause vomiting, abdominal pain, bloody diarrhoea, nervous system stimulation, depression, trembling, paralysis and cardiac arrest.
Large quantities of salt can lead to an electrolyte imbalance.
This is another food which is fine to feed as an occasional treat, but mustn't be fed regularly, as it can lead to a condition known as Steatitis or Yellow Fat Disease.
This can expand in the stomach causing bloating and discomfort.
Other foods which your cat shouldn't eat:
Soy and Rice Bran
Spoiled or mouldy food. If it's gone off, don't give it to your cat.
It is always a good idea to check with your vet before giving your cat food which is meant for human consumption, just to be on the safe side.