A gene got expressed in a wrong place... that's a mutation... then came selection... :-)
My thoughts exactly, though I was under the impression that venom-producing
tissue was derived from enamel-producing pits along the snakes' jaws. That
got me doubting that the gland venom was itself a derived salival gland,
though on hindsight the enamel-producing tissue could have been expressed or
migrated elsewere during development.