Yep, you're right, but it's usually not your main brands:

"Krush Nutrition's "Nutty by Nature" xylitol-containing peanut butter. Xylitol, a natural sugar alcohol sweetener popular for its low glycemic index but known to cause hypoglycemia and hepatic necrosis in dogs, is now also found in several specialty peanut and nut butter brands."

Xylitol is the same stuff that's in some chewing gum that is super deadly for dogs.

One nice thing about big dogs is you get more leeway in your parts-per-million on accidental toxins. Those two whits dogs knocked a bottle of chocolate liquer off of a refrigerator and slurped most of it up. Spent a long time stuffing activated charcoal down their throats until they threw it back up.