On the one hand we know that if a Sanhedrin issued an erroneous decision (which was acted upon) they must bring a sacrifice (Vayikra 4:13).
On the other, we have the concept of "lo bashamayim hi" (see also: Bava Metzia 59b) namely that the meaning of Torah is decided not in Heaven, but by Rabbinical interpretation here on Earth. And Sanhedrin seems like the ultimate interpreting body.
So how then can the Sanhedrin ever err? I'm not talking about errors in matters of metzius (such as, husband went away on journey, they thought he was dead, but he came back alive). Rather, I mean errors in interpreting the Torah - what should be permissible and what should be forbidden. It seems that whatever the Sanhedrin decides, is correct or becomes correct, by definition (due to "lo bashamayim hi").
If we say they can err, that implies the existence of an independent standard of truth, against which errors can be made. Which would go against "lo bashamayim hi".