Quote Originally Posted by Vela
Actually that isn't completely true that it's like chocolate, grapes and moreso raisins, can be toxic in different quantities depending on the dog. Some dogs can die from quite small quantities depending on the dog and if it has any underlying unknown possible problems excreting the cyanide. It is also not known if the effects are cumulative over time.

Onions, grapes/raisins, avocados, the pits of fruits (especially large ones like peach or mango pits, but even apple seeds contain small amounts of cyanide).

http://dogs.about.com/cs/disableddogs/a/poison_food.htm
Thanks for correcting me. I don't know a whole lot about why things are poisonous to dogs, I just know what not to feed and I avoid it