Angelo Hays was buried after a motorcycle accident in France in 1937.
The insurance company were unhappy for some reason and insisted on his exhumation several days later. He was found to be alive, but deeply unconscious. He made a full recovery.
A German woman woke up in a coffin after a traffic accident, having been pronounced dead.
There have been many such incidents so it is highly likely that people have been buried or cremated whilst in a deep coma.
Embalming is not routine, and is not necessary these days, unless the funeral is delayed for any reason.
The Irish newspapers reported the arrest of a naked man, Peter Archer running down the street in 1996. He was let off when he explained he had been fleeing from a morturary, having officially been pronounced dead.