This (a permanent revive) has happened to me about three or four times
over the last five years.  So it seems it requires a chain of events that
does not happen very often.  I do remember that when I got this last one,
I had revived one or two too many, so one or two of the ones that were
there crumpled when the new ones appeared.  Whether this contributed, me
no know.  Which is why I was hoping somebody here knew more about this.
I have gotten Fallen that could not hit me, or be hit by me, much more
often, and I am still not sure how that happens either, although I think
it has something to do with killing the Shaman just as he is re-raising
that particular Fallen. :\