This case has happened before, but it is still weird when it happens. Except for some extra-ordinary cases where a person has supposed to be declared dead and then wakes up later, it is normally pretty easy for a doctor to determine if a person is dead (pulse, heart, brain activity), and unless revival has happened, the person stays dead. Thus, it is weird if the person wakes up after being declared dead (link to article):
In what could have been a scene from a comedy, a patient at Sion Hospital woke up minutes before his post-mortem, after a doctor had mistakenly declared him dead. The reality is far from funny, however, as the incident points to shocking negligence at the civic hospital. What's worse, according to the police, the doctor at fault immediately destroyed all records of the incident in an effort to cover up his mistake. The body was taken to the mortuary on a stretcher, where it was kept near the lift in the lobby. From there, the body was to be taken to the first floor for the post-mortem. But just as morgue staffers Subhash and Surender were about to take the body upstairs, they received a shock: "The dead man was breathing. We saw his stomach moving up and down," said Subhash.