If a brother did ma'amar with his brother's yibum, and then divorced that Levirate betrothed, leaves the possibility of yibum for the third brother, with the co-wife. But the woman who had ma'mar and then a get is in a complicated position. (And corresponding other cases). Plus a new mishnah, with another case (well, set if cases) - and bumping up against the claim that the mishnah isn't supposed to be changed. Also, the difference of approach between R. Yosei and R. Shimon, on eisher ach vs achot ishto. Which boils down to one prohibition following another prohibition, and the question is which wins, or does one?