Tapioca.
Tapioca, if not processed properly, can be toxic. It contains a natural source of cyanide, whose precursor, linamarin, is rendered harmless if properly dried, soaked, and baked.
Blowfish (Fugu).
This fish is deadly on its own, and highly regarded as a Japanese and Hong Kong delicacy. It’s poison, tetrodotoxin, is 1,200 times more potent than cyanide. Around one hundred diners die each year from it. Preparation is the key; make sure a licensed chef prepares your Fugu. Becoming a licensed Fugu chef is a long and intensive process, culminating in the chef preparing a Fugu meal and eating it himself. The most deadly parts of Fugu include the liver, muscles, ovaries, and skin.
Mushrooms.
Shellfish.
Rhubarb.
Potatoes.
Apricot Stones (Seeds).
Tomato.
Chocolate.
Tapioca.
Peanuts.

This list has been severely abridged. To read the full list, view the original post at it’s source:
Dangerous Foods (DivineCaroline)