Chullin 95 - Meat Out From Sight
Rav said, "Once a piece of meat was out of sight, it is prohibited - perhaps a raven switched it with a non-kosher piece."
But previously we learned that one is allowed to buy meat from non-Jewish resellers!? Rav will answer that perhaps the reseller never lost sight of the meat. Nevertheless, it was Rav's own stringency.
Rav was once going to the house of his son-in-law and saw a ferry coming toward him. He said, "It is an omen that there will be a festival where I stay." When everyone came to great him, he himself kept watching the meat. Then he said to the butchers, "If not for me, you would have fed prohibited meat to my family!"
After all, Rav himself did not eat that meat. Not because it was not watched - for he watched it, and not because he used ferry for divination, but because it was a wedding of a daughter of a Sage to an unlearned man, and a Torah scholar like Rav is prohibited to eat at a feast that is not by itself a mitzvah.
Art: Edward Frederick Brewtnall - The three ravens