Pesachim 80 - The forgiving power of the tzitz

Tzitz is the golden head plate worn by the High Priest. It fixed the defects of sacrifices, but not all the defects. Thus, if someone brought his Passover offering and even threw its blood on the Altar, but then they discovered that the meat or the blood became ritually impure, it is the tzitz that effects the acceptance. What is the use of acceptance if the meat cannot eaten? At least, he has fulfilled his obligation and won't have to bring another sacrifice. He will eat matzah and other things at the Seder.

What if they discovered that he himself was ritually impure? Here the tzitz does not help, and he will have to bring the Passover offering a month later, on Pesach Sheni. However, here is a special exception. If he was impure due to the "impurity of the depth," his Passover offering is valid, and he will not have to bring another one - all due to the tzitz. What is this special "impurity of the depth?" - This is something that absolutely nobody knew about, such as a corpse under the house, unknown to anybody, that was discovered later.

Art: Study of the Head of a Corpse by Charles Emile Champmartin