The Torah teaches (Shmot 28:36-38) that one of the parts of the uniform of the kohen gadol – the high priest – is the tzitz, a golden plate on his forehead. As understood by the Sages this tzitz has the power to remove responsibility from the kohanim in the event that a sacrifice was brought and it turned out, under certain circumstances, to have become tameh (ritually defiled). One of the cases where the tzitz accomplishes this appears in our Mishna (80b) – if someone who had brought his korban Pesah discovers that he was tameh from tumat ha-tehom, he does not need to bring a korban a second time on Pesah sheni, since the tzitz allowed the original korban Pesah to be acceptable even though its owner was tameh. Tumat ha-tehom is a case where the dead body that is giving off the tuma is hidden in a place where no one knew it was, and it was only discovered after the person who came into contact with it had already brought his korban. Although the Gemara on our daf tries to find a source in the Torah for this halakha, its conclusion is that there is no clear reference in the Torah for it, rather it is a halakha le-Moshe mi-Sinai, a law that was transmitted orally to Moses on Mount Sinai that was not recorded in the Torah.
The Gemara on our daf tries to use this rule as the source for other cases. Would the same rule of tumat ha-tehom apply to a kohen who is bringing a korban Tamid, the daily sacrifice that opens the sacrificial service in the morning and closes it in the afternoon? Although Rabbah tries to apply the rule of kal va-homer (a fortiori) to this case that would allow the case of korban Pesah to be a source for other halakhot, the Gemara rejects this, arguing that we cannot learn a kal va-homer from a halakha le-Moshe mi-Sinai.
Although we usually perceive the rule of kal va-homer as being a straightforward logical one, it cannot be used in the case of halakha le-Moshe mi-Sinai because of the unique quality of such halakhot. In general, a law that appears in the Torah can be used not only for itself, but also as a source for other laws that can be compared to it. A halakhah le-Moshe mi-Sinai, even as its strength and severity are equal to those of a law written in the Torah, is not seen as being grounded in the same set of rules as the written halakhot, so we cannot extrapolate other laws from it.