Could cause little reaction, or death if vital tissue is involved. Accidents happen, as when a strike badly misses target prey. Then there are those stories of cornered or injured snakes biting themselves to commit suicide. Certainly a snake under stress may self-inflict, most notably when it's pinned with a hook or stick and goes into a frenzy. Brought into a TV studio, a sidewinder rattlesnake began self-biting and died shortly afterward.
Extreme cold is another factor.
On several instances I found rattlesnakes with their fangs embedded mid-body after euthanization. The Beacon Journal Publishing Co.