Plan as you continue to go
If you don't want your cat to scrounge food from your plate then don't ever give him tidbits. It may seem cute to occasionally indulge him in this way, but it encourages begging behaviour, which is highly likely to become ingrained behaviour. So, if you don't want your cat to learn that it is acceptable to expect food from your plate, don't do it. This also applies to allowing your cat to jump up onto benches or the dining table. Not only is it a bad habit to allow for hygiene reasons, but it is also potentially dangerous. If your cat learns that it is acceptable to jump up and take food, it may eventually eat something which is either poisonous to it (many human foods are poisonous to cats) or injures itself on a cooked bone.
Obesity has become a huge problem in cats over the past 20 years. There are a number of life-threatening diseases which go hand in hand with obesity. It is easy to show how much we love our cats with food, but this can't be allowed to happen. Obesity has an impact on our cat's quality of life as well as lifespan and I am sure none of us want harm our cats in any way.
Feeding human food to your cat
If you do want to give your cat human food, don't make a habit of doing so as it isn't nutritionally balanced for the cat. The occasional table scrap is fine if only given very occasionally, and always put the food into the cat's bowl. Also bear in mind that if you feed human food regularly, it may make your cat picky and less likely to want to eat his own food.
Feeding food past its use by date
Cats are not waste disposal systems. If the food is out of date, or has spoiled and isn't fit for human consumption, then it is not acceptable to expect your cat to eat it. Cats can become sick with food poisoning just like humans can. If in any doubt whatsoever, don't give it to your cat.
Feeding dog food
Cats have different nutritional requirements to dogs and feeding them dog food will result in a taurine deficiency. Dogs eat dog food and cats eat cat food.
Too much of one type
This mainly refers to people feeding a home made diet. Unless you are well versed in feline nutrition then it is best to feed a commercial cat food which has all the necessary nutrients in it.
Two foods which can cause problems are tuna and liver. Tuna can cause steatitis (yellow fat disease) if given too often and liver can cause vitamin A toxicosis (cod liver oil can also cause vitamin A toxicosis and therefore should be avoided). So, while it is fine tuna and liver to give them to your cat, make sure you do so in moderation.
Feeding a diet with lots of raw fish
Raw fish contains the enzyme thiaminase which destroys thiamine (Vitamin B1), resulting in thiamine deficiency. Cooking destroys thiaminase, thus protecting thiamine.
Putting your cat on a diet without veterinary supervision
Dieting in cats, especially obese cats should be done under the supervision of your veterinarian. Weight loss needs to be done very slowly in order to prevent your cat from getting Feline Hepatic Lipidosis (fatty liver disease).
Feeding foods which are toxic to cats
Many pet owners make the assumption that because a food is safe for human consumption it must be safe for our cats to eat. This isn't the case, and there are foods which are toxic or dangerous for our cats to eat. Chocolate, for example, contains a substance called theobromine which is extremely toxic to cats, grapes and raisins contain an unknown toxin which causes kidney damage. Onions cause a type of anaemia.
This is another reason why it is always inadvisable to feed your cat table scraps and tidbits off your plate. For a more extensive list of foods which are dangerous for your cat to eat, please read here...
Putting a cat on a vegetarian diet
Cats are obligate carnivores, they must eat meat to survive. If you are against eating meat for moral or ethical reasons then that is a choice you are able to make. As humans, we can get along just fine without consuming meat. Cats cannot, so please don't try to make your cat eat a vegetarian diet.
It's tempting to share food with our favourite feline companions but it is not a good idea to make a habit of offering them snacks as this can lead to obesity. Also, if you are offering them a bite or two of your turkey sandwich this may decrease their appetite when it comes to meal time, making them eat less of their nutritionally balanced cat food.