The halacha states that in order to punish a person who has violated the laws of the Torah, one must first give hasra’ah, a warning. However, in this week’s Parsha, Chazal learn that this does not apply to Bnei Noach.
Hashem says to Avimelech that he will die as a result of his taking Sarah, who was a married woman, and we do not find anywhere is the passuk that he was warned.