Hullin 10: The Possibly Non-Kosher Shechitah of 13 Animals

Given that the knife for shechitah cannot be notched, there was an event when a knife was discovered to be notched after it was used to slaughter 13 animals. But if he only discovered that to be the case after the last animal's shechitah, then maybe it became notched along the process and the first animal(s) were not a problem. The big - and perhaps indeterminable - question is when did the knife received its notch? All of which leads to the conclusion that the knife must be checked (for notches and whatnot) before each and every act of shechitah. Also, investigating the source of "chazakah" (the given status of a thing unless or until proven otherwise). Beginning with the example of tzara'at, in the case of a house. But what if that tzara'at is behind the door? Or in a dark house? The question of whether there's a tzara'at lesion to find does not require spotlighting the whole house in the search! Note that the source of establishing a given is in the context of tzara'at, and not kashrut, but it's not the first time they are linked.