Pesachim 71 - Trying to do a mitzvah but failing

One should slaughter a Passover lamb on the Passover day, even if it falls out on Shabbat - when the slaughter is usually forbidden. However, if his slaughter is invalid, he has transgressed Shabbat, albeit retroactively, and now he needs to atone for it by bringing a sin-offering after the Holidays. For example, this happens when he takes a Passover offering and brings it for the sake of peace offering, which makes it completely invalid.

However, if he takes a peace offering and slaughters it as Passover, he has done at least some sort of a mitzvah, and Rabbi Yehoshua says that he does not need to bring a sin-offering. 

If the animal had a blemish (which invalidates any offering), he should have checked it before the slaughter. That is negligence, and even Rabbi Yehoshua says that he is liable to a sin-offering. But if it is invalid due to an animal's internal defect, then everybody agrees that there is no sin-offering because it was like an unavoidable accident. The meat is not kosher and cannot be eaten, but he is not to blame.

Art: Butcher’s Stall by Pieter Aertsen