There are a number of mumim – blemishes – in a sacrifice – that would preclude the animal from being brought as a sacrifice (see Sefer Vayikra 22:24).
The Gemara quotes a baraita that teaches the opinion of Rabbi Eliezer that if parts of a valid sacrifice become mixed with parts of a sacrifice from a blemished animal, they can, nonetheless, be burned on the altar, and the parts of the blemished animal will be viewed as wood, i.e. as fuel for the fire, rather than an actual sacrifice. Ultimately, the source brought for Rabbi Eliezer’s opinion is the continuation of the passage in Vayikra (22:25) where the Torah emphasizes that when the blemish is in the meat it cannot be brought, but in a situation where there is a mixture and the mum is not in all of the sacrificial meat, then it would be permitted on the altar.
In the course of discussion, Rav Huna suggests that this may follow the opinion of Rabbi Akiva who permits certain types of mumim to be brought on the altar, or at least that the sacrifice is not removed from the altar if it had already been brought to it. The case discussed is when the blemish was on the dukin she-ba-ayin. Dukin she-ba-ayin is the cornea of the eye; Rashi explains that the blemish is an unobtrusive one, such as a cataract on the eye. According to the continuation of the Gemara, Rabbi Akiva’s position is explained by Rabbi Yoḥanan as being limited to this case, for although dukin she-ba-ayin refers to an actual blemish, since in some types of sacrifices – notable sacrifices brought from birds – it is not considered a blemish, it would not be removed from the altar.
It is interesting to note that in the famous Kamtza-Bar Kamtza story that the Gemara presents as the direct cause of the destruction of the Second Temple (see Massekhet Gittin daf 55), Bar Kamtza convinces the Caesar that the Jews were rebelling because they refused to sacrifice the animal that he sent to the Temple. Bar Kamtza succeeded in doing so by making a blemish in the dukin she-ba-ayin of the animal, which, the Gemara explains, was considered a blemish by the Jews but not by the Romans. In that Gemara, Rabbi Yoḥanan is the Sage who objects to the Rabbinic refusal to consider sacrificing the animal in order to keep peace with the government; Rabbi Akiva objects to Rabban Yoḥanan ben Zakkai’s refusal to appeal to Vespasian to save Jerusalem and the Temple.