As we learned on yesterday’s daf (=page) when tithing animals, they are brought into a pen and a small opening is made so that two animals cannot leave at the same time. The owner counts the animals one by one, marking every tenth animal with red paint announcing “this is the tithe.” The Mishnah on today’s daf discusses cases where two animals manage to exit the pen at the same time.
Generally speaking, if a pair of animals leaves at the same time, the person counting simply counts them as two animals. In the event that he counted the pair as a single animal, this would create a halakhic problem for the animals that are counted as number nine (which is really the tenth animal and therefore the tithe), and as number ten (which is not the tithe, but simply the eleventh animal).
What if the ninth and tenth animals exit the pen simultaneously?
Although the version of our Mishnah that appears in the Gemara does not discuss this case, it does appear in some manuscripts and it is clear from his Commentary to the Mishnah that the Rambam had this version, as did a number of other rishonim. The Tosafot Yom Tov explains that according to this reading, given that it is impossible for both of the animals to have exited at exactly the same instant, we cannot declare that both of them be considered to be tithes. For this reason, one of them must be the tithe while the other is an ordinary animal; our problem is that we cannot tell which is which. Since we cannot sacrifice the ordinary animal, we have no choice but to allow them to graze until they develop mumim – blemishes which preclude their sacrifice – at which point they can be slaughtered and eaten by their owner.