If

[Little Rik]

Because we know that people make mistakes... and doctors are people. And, usually, the diagnosis of death doesn't get peer reviewed, so... yeah.
Also, "proven"? Usually, a doctor declares a patient as dead, not proves the patient is dead.
To prove a patient is dead, they'd have to run some sort of a brainscan... or wait long enough - typically, 5 minutes without oxygen is enough for braindeath to be declared with very little margin of error, but 10 minutes is even better!
Still, there's a likelihood of error.

Gee, i always knew that the doctors are all a bunch of idiots.
With only 6 or so years at the uni they think they know everything.
They really should take advise from the atheists in order to know how to declare the guy alive or dead.

So the guy maybe wasn't dead to begin with, just weak... very weak... unnoticeable heartbeat... no motion... and then the person wakes up, claiming to have seen or felt something while in that low powered state.
You attribute that newly formed memory to NDE.... we say it's much much more likely to be just dreams.

Of course that they all were dreaming.
Thousand of them.
All in a zombie state.

http://www.nderf.org/NDERF/NDE_Archives/NDERF_NDEs.htm