There are unfortunately regular deaths of dogs from chocolate Toxicity. Chocolate contains Theobromine, a Methylxanthine compound which in large amounts can cause increased heart rates, cardiac arrhythmias, arrest and death.
The poisoning effect is weight related, which is why it is the Toy Breed dogs that are most at risk. A 3 kg Chihuahua would only require one whole small 60g bar of dark chocolate to be at serious risk of collapse. A 40kg Labrador would be at risk of poisoning if it has knocked back a king size block.
Any dog which has eaten a large amount of chocolate relative to its body weight needs immediate veterinary care. A general ratio to consider if you are concerned your dog has eaten too much is 20g of chocolate per kg.