I remembe when my cousin came to stay with me when we were 14 and we used to stay up late at night, watching horror movies and scaring ourselves. There was a movie about someone with Catalepsy who was buried alive - it was only fiction but it scared us stiff.
It was something which terrified my grandmother - the Victorians seemed to havea ghoulish delight in these things. She used to tell me tales of people who'd been buried alive and tried to scratch their way out :-(
At least with being cremated, there is no way back if you were in some sort of coma.....
Seriously (honestly...) I think that the palaver a body has to go through with undertakers/PM's/Coroners officers these days it would be impossible for anyone to slip through whilst still alive.
My Mum was buried alive, and dug out as a child, when her house was bombed during the war.
She says that she is NEVER going to be buried again, has told us all, ready for when something happens to her.
She has even gone as far as leaving strict insructions that she must not be buried with her solicitor.