3 interpretations of the most recent mishnah: Shmuel maintains that the atonement kicks in when the blood is appropriately on the altar. Resh Lakish, however, says that if the offering is invalidated, the atonement doesn't work, but there is still room for Shmuel's understanding about atonement. A third understanding is R. Yohanan, which is that there's no atonement. Note that the Gemara does relate the views to each other. Also, a shift to focus on the burnt-offering, and Rabbi Shimon's understanding of the details of what invalidates such an offering - with a comparison between the mishandling or error in transferring or placing the blood vs. an issue with the suitability of the animal itself (for example, and this is the Gemara's first example of a problematic animal, the animal used in bestiality).