Grapes and raisins, for reasons that are not understood, can be lethal to dogs — causing irreversible kidney failure. Some dogs can eat them with impunity; for others even a single grape can be lethal. The toxicity is not breed-specific. You don’t want to find out the hard way whether your dog is susceptible!
Knowing about the phenomenon, I have always tried to be super-vigilant when walking my dogs, and keep them away from any stray grapes. A couple of days ago someone had pruned a backyard grapevine and discarded the prunings — full of over-the-hill grapes halfway to becoming raisins — in a leaf pile by the gutter, where they were utterly invisible. But my dog sniffed them out, and I had to drag her away from what could have been her last meal.
This sounds like an urban legend, and most dog owners are unaware of it — but it’s true. Please, if disposing of grapes or raisins in any form, do so in a manner that will keep pets out of harm’s way.