A quick list of DO NOT FEED here, I would also like to add that there are many toxic houseplants and chemicals we use around the house, be sure these things are out of reach of your pet rat.

Green or not ripe bananas
Cause stomach upset and prevent starches from being digested

Blue Cheese
This and most other molds are toxic to rats

Raw/Dry beans
Should be cooked before eating, otherwise toxic and causes blood and stomach problems.

Dried Corn
Contains fungus that may cause liver issues or cancer.

Wild-caught insects
May have parasites, diseases, poisons or toxins.

Black Licorice
Neurological symptoms

Green potato skins, eyes that are growing
Toxic (to people as well)

Raw Sweet Potato
Can produce cyanide in the stomach when digested.

Raw Peanuts
Roasted are fine

Apple Seeds and other Fruit pits or seeds
Can produce cyanide when digested.

Rhubarb

Anything thick and sticky like nut butters, peanut butter, should be spread thinly on something or mixed with something to make it less sticky. It has been reported that some rats will choke on peanut butter if they take a large bite of it.