Here's a list of unsafe food for rats i've pinched from the fanciers forum. Posted by Dimruthien.
Alfalfa - contains canavanine which can cause hypersensitivity illness
Avocado - skin and pips are toxic
Alcohol - toxic
Beans - Lima and Kidney in particular, but I'd avoid all uncooked beans. In an uncooked state they contain cyanide
Brussel Sprouts, Cabbage, Kohlrabi - All can cause thyroid problems (goitre)
Canola (and canola products) - comes from rapeseed, which is toxic
Carbonated beverages - Way too sugary, even diet sodas contain artificial sugars
Cottonseed Oil - it is suspected to cause illness, but as yet is unproved. I'd probably avoid it anyway!
Comfrey - can cause bladder tumours
Fruit Tree Leaves - toxic, and can cause a range of illnesses and symptoms
Fruit pips - contain cyanide
Green Potatoes - toxic
Medicinal Herbs - can be toxic, and can cause allergies
Licorice, Salmiakki - can cause high blood pressure, salmiakki has too much salt
Margarine - can be toxic, and also often contains canola
Millet - contains cyanide
Morning Glory - is a hallucinogen
Nightshades - toxic. the nightshade family also includes potatoes, tomatoes, eggplant. it's a good idea not to feed rats leaves, just in case!
Nutmeg - is a hallucinogen
Orange rind - carcinogen, particularly in males
Peanuts - can contain mould
Peanut Butter - choking hazard, sometimes contains rapeseed oil
Periwinkle/Myrtle - is a hallucinogen
Rhubarb - toxic
Spinach - toxic
Sweet Potato/Kumara - can often have some degree of fungal growth, it is fine peeled and cooked
Artificial Sweeteners - are mostly toxic but it is safe to use Stevia and honey
Wild Insects - can carry parasites, bacteria, etc. much safer for your rats to buy insects at the pet store (or breed your own)