Let's get started... many things with feeding chickens are no brainers, but other items on this list my shock you, that's why its important to read-up and do your research before start feeding them potentially poisonous treats. I thought about making a top-ten list, but these are all very bad for your birds and should not be taken lightly.
Some "no brainers" are; Alcohol or tobacco products. Human medicines, bio-degradable plastics and paper. They may seem silly, but people have fed these things to their birds. Some should probably reconsider keeping animals period.
Raw Potato Peels are toxic. Avocado skins and pits are toxic. Chocolate is also very toxic. Junk food has a high fat and salt content, non of which are good for these small creatures.
Moldy foods, you may have heard they are OK to feed to your chickens, but unless you're a mold expert, you never know what spores or toxic molds might harm your chickens. Sugar is just bad, it elevates the blood sugar and gets the heart racing a dangerous levels. Garlic and onions can effect the egg taste as well as be toxic. Raw meat, just don't, there's a high risk of bacteria, smell and food poisoning.
Salty foods can make your birds sick. Your average garden clippings can contain toxic or poisonous plants that your birds wouldn't know better then to eat. Feeding raw eggs may cause your birds to eat their own eggs as they now see them as a food source. Be mindful to crush up any egg shells into tiny tiny pieces before feeding back to your flock. Feeding raw beans can be dangerous because they can expand while being digested.
Other unwelcome items include:
Banana and citrus peels, certain flowers, parsley, anything really spicy, dried fruit, sugary cereal, be careful with some wild bird seeds, cheeses, raw rice and raw pasta. Feed scratch corn only in the winter months as it elevates their body temperature, avoid in warm months. Some people like to feed cooked meats and even cooked chicken... but this seems really messed up and cannibalistic.
For more information on what not to feed chickens, check out your local library for a nice big book on chickens or a credible web site, like a University's agricultural department.
Another thing to keep in mind that uneaten kitchen scraps in your coop may start to smell and get moldy, so be sure to remove uneaten waste and clean regularly, food might collect droppings and birds will eat and track around. Extra waste may also attract pests and predators.