Not just anybody can pronounce a person "dead." In fact, "death" requires medical diagnosis by an M.D. (in other words, a person with roughly 11 years of training) or, depending on the jurisdiction and the details of the death, a paramedic or a registered nurse. Even so, medical professionals occasionally make mistakes, declaring people dead who really aren't. If a person is hypothermic, or has taken barbiturates, they may appear to be dead on examination. There are certainly reported cases of humans waking up in the morgue or on the embalmer's table, and it is a deep fear of being mistaken for dead that drives the invention of caskets with built in alarm systems in case the deceased wakes up underground.
Could this also happen with our companion animals? Could we mistake them for dead, when they really aren't? Particularly if we have chosen to euthanize, which is generally accomplished through an injection of barbiturates? Indeed, yes. It can happen. Take, for example, the case of Mia. Mia was a ten year old Rottweiler whose family, after watching her suffer from a crippling arthritis, decided to have her euthanized. After the vet administered the standard two-injection protocol, Mia's mourning owner took her dead body home. She was placed in the garage overnight, to be buried the next day. Imagine the owner's shock when Mia greeted him at the garage door in the morning. Now, there was much hand-wringing the in pet-media over this case, and many reports of similar not-so-effectively euthanized dogs, and the obvious question: How do we know this doesn't happen all the time? Usually, an animal's body is placed into a freezer just after being euthanized-until the scheduled pick up from the crematory service (usually once a week, which is too infrequent just to leave bodies lying around). So, really we would never know. Which may make your hair stand on end just a bit.