Has a doctor or somebody ever made a mistake saying the person was dead(when they were still alive) and people buried him alive
Yes, but who knows if it was a "mistake". Coffins have been found to have scratch marks. Though there is a process using formaldehyde... where they drain the fluids and blood first. So hopefully you would wake up by then.
Stephen King has a ahort story about this type of situation in his book Nightmares and Dreamscapes, it's on DVD as well and it's pretty intense!
Just last week in south America a parent came into morgue to see their dead baby who was alive when they were shown the baby