One of my dogs ate a nectarine pit but the others only had the flesh — what's the risk difference?

The risk is significantly higher for the dog that ate the pit. While the flesh is safe in small amounts, the pit contains cyanide precursors and is a serious choking or intestinal blockage hazard. The dogs that only ate flesh are at low risk, but watch for any mild stomach upset. For the dog that swallowed the pit, you need to monitor closely for choking, vomiting, or signs of abdominal pain, and contact your veterinarian immediately for advice.