the whole notion of hacha bemai askinan, and similar situations, where the gemara relegates the halacha stated in the mishna to a very unusual case, has always troubled me. it is difficult to accept that this is the case that the mishna really intended. do you have any insights to this.
dmartin, raanana, eretz yisroel
In most cases, such as this one, quite the opposite of what you write is true. The Mishnah is actually giving us the general rule, which is implied by the Halachah; in this case, it is that the Minchah of a woman who was Mezanah must be burned. This is the basic Halachah as it applies to the Halachos of Sotah. However, the Gemara says that practically this Halachah cannot come into play, because of a Halachah in Hilchos Menachos with which the Mishnah here is not dealing. As long as we can then find a case where the Halachah of Sotah applies, then the Mishnah is not redundant, and it has taught us a Halachah with regard to Sotah.