By the way I was kidding when I said “But I who do know kol hatorah kulah and couldn’t answer this kashya for sixty years, can entertain the possibility that the Ri gave an impossible example.”
But I will say that there are different types of mistakes. The potential mistake in the yibum case would be a basic mistake in cheshbon. Which is a much bigger mistake then say, forgetting an obscure source. Maybe not Rishonim, but for sure by acharonim, whereas I would be very hesitant to say that they made such a basic cheshbon mistake, I don’t think it would be out of the realm of possibility to say that they missed a source. Now perhaps missing a source might not necessarily be called a mistake in the first place. Also, I wouldn’t say that we don’t assume that Rishonim made mistakes because we have such a mesorah. I would say that we assume that they didn’t make such misakes because it is clear to anyone who reads Rishonim that they were brilliant and knew probably every gemara and were experts in cheshbon. The amount of times that I’ve had questions on Rishonim in cheshbon and later realized I was wrong, is more than enough to tell me not to treat them lightly.