Cocoa mulch, which is sold in hardware stores and garden supply stores, contains an ingredient called 'Theobromine,' which can be lethal to dogs and cats. It smells like chocolate and is especially attractive to dogs. Depending on your pet's size and the amount of mulch it eats, it could be life-threatening. 

Theobromine is in all chocolate, especially dark or baker's chocolate which is also toxic to dogs. Cocoa bean shells contain potentially toxic quantities of theobromine, axanthine compound similar in effects to caffeine and theophylline. One dog that ingested a lethal quantity of garden mulch made from cacao bean shells developed severe convulsions and died 17 hours later. Analysis of the stomach contents and the ingested cacao bean shells revealed the presence of lethal amounts of theobromine.

If you suspect your pet has ingested ANY sort of poisonous substance, seek the help of a veterinarian IMMEDIATELY!

You can learn more about the danger of cocoa mulch in this posting at About com or at the Snopes.com website. In addition, the Humane Society of the United States has information about a variety of items and substances  commonly found in the home that can be potentially dangerous to pets.