He has to bring a korban (offering). In the previous Mishnah, we quoted a Mishnah from Horayot that said that if the Bet Din (rabbinical court) makes a mistake and the majority of the Jewish people follow that mistake, then the Bet Din has to bring a special korban. But we brought an exception to that rule: a mitzvat ase sheba Mikdash she'ein chayavin aleha, a positive commandment in regards to the Bet HaMikdash. If the Sanhedrin gives a ruling according to it, and it is a mistaken ruling, and the majority of the Jewish people violate it, then in that unique scenario we say that the Sanhedrin is not obligated to bring the special korban as a make-up for that mess-up by the Sanhedrin.
We're going to continue with that Mishnah, which brought a second din (ruling): the Sanhedrin is obligated to bring a special chatat (offering) if they rule about a positive commandment concerning a nida (menstruating woman). The Mishnah says, “What's an example of the positive commandment concerning a nida for which the Sanhedrin is obligated to bring a special korban if they make a mistake?” We say it's the commandment that applies in the following halacha (law).
If a man was having relations with a tehorah (pure) woman who wasn't a nida, and while they were in the middle of having relations she said to him, “I have become tameh (ritually impure). I felt that I had become a nida,” then, if the man withdrew from her immediately and still had an erection at that point, he has to bring an offering because when he leaves her—when he exits her—it's also a pleasurable movement. When he moves in and moves out, every movement is considered pleasurable for the man. So what he has to do is wait for his erection to subside, and then he can remove himself from his wife. That's the only way he's allowed to withdraw. If the Sanhedrin makes a mistake about this ruling and says that the man should withdraw immediately, then they're obligated to bring the special korban chatat offering. They told the wrong ruling, telling husbands that they're allowed to withdraw immediately, when in reality any movement is considered pleasurable, and he would be having pleasure from a nida, which is a prohibition.