I grew up giving chocolate to dogs. My boxer, when I was young, would share my Halloween candy. One for me, one for the dog was the usual arrangement. Never seemed to do him any ill, though I know it can poison them now. My beagle who died about three years ago used to LOVE grapes. HE liked to peel the skin off first, then eat the rest. Again, I never knew it was bad for him, though a few times he got entire bowls we forgot to pick up. Never seemed to do him any harm.
Hadn't heard about the onions. Or the garlic. My dogs eat both, guess I should stop giving it to them.
I am told the sugarless gums containing Zylitol(orbit, stride, etc) are especially lethal for dogs. Not specific to gum, actually, it's the Zylitol.