Chullin 101 - The Sciatic Sinew of a Non-Kosher Animal
The prohibition of a sciatic sinew applies to a kosher animal, but it does not apply to a non-kosher one. What difference does it make, if a non-kosher animal is anyway forbidden to be eaten? One is transgressing a different prohibition when eating it - the prohibition of a non-kosher animal, and not the one of a sciatic sinew. That is important for giving him a proper warning, that is, telling him exactly what prohibition he transgresses, moments before he eats it, as a prerequisite for lashes.
Rabbi Yehudah disagrees and says that the sciatics sinew of a non-kosher animal is also forbidden - because it was prohibited since the time of Jacob, when non-kosher animals were still permitted. The Sages replied to Rabbi Yehudah, "The sciatic sinew prohibition was first given to Moses on Sinai, and it did not apply before then. However, when Moses recorded and arranged the Torah, he recorded this prohibition in the context of Jacob's encounter with the angel, to teach that this incident was the reason for the prohibition."
Art: Andrea Commodi - Young Woman in the Kitchen