The point at which "a programmed system begins to program itself." Metaprogramming begins in that split second when a pre-scripted program consciously begins to rewrite itself, setting aside the reward/punishment systems it was imprinted with.
In humans, this means "real" rebellion. To go against all you have been taught and truly start making decisions for yourself.
Indeed, the act of metaprogramming may be the ultimate benchmark of a conscious system - I metaprogram, therefore I am.