A new mishnah! What items would be taken off the altar if they were brought there, even if by accident? Plus, what parts of an animal that was brought for an offering were not burned with he rest of the animal - for a burnt-offering, that is? Moreover, the Gemara addresses the principle of burning the offering in full. But is everything really burned in full? And... another new mishnah! With a focus on items that were up on the altar, and then something that is there is dislodged from the spot -- well, then, that next part that fell is not returned to the altar. Plus, the question of timing - when something is on the altar and is burned however much, and then it falls... the Gemara clarifies how much time is left for the burning will determine whether the item is returned to the altar, with both the before and after midnight cases (and an explanation as to why both are necessary).