I attended an open-casket eulogy in little town in Oklahoma years ago. It was a small church with only the front doors for entering and leaving.
For some reason the corpse started slowly `sitting-up' during the service! There were several `exits' after that happened, but we proceeded to the cemetery after things calmed down.
The problem here may be that the ground just won't accept this bird . . .
Rigor mortis.
I knew a nurse who said that she and her buddy witnessed this in the morgue. They beat a hasty retreat. 8-)