Rabbinic Fallibility (Bava Kamma 99b)

This need not be said, except for those who insist on a religious doctrine in which rabbis are such experts in even non-Torah matters that they can make no mistake.

Bava Kamma 99b:
There was a certain woman who showed a denar to R. Hiyya and he told her that it was good. Later she again came to him and said to him, 'I afterwards showed it [to others] and they said to me that it was bad, and in fact I could not pass it.' He therefore said to Rab: Go forth and change it for a good one and write down in my register that this was a bad business. But why [should he be different from] Dankcho and Issur42 who would be exempt because they needed no instruction? Surely R. Hiyya also needed no instruction?

We see here that Rabbi Chiyya could have made an error. So too on the next daf (Bava Kamma 100a):

Resh Lakish showed a denar to R. Eleazar who told him that it was good. He said to him: You see that I rely upon you. He replied: Suppose you do rely on me, what of it? Do you think that if it is found bad I would have to exchange it [for a good one]?

We see that Resh Lakish would consult another to obtain the metzius, rather than relying on his own omniscience. And he knew that even if Rabbi Eleazar told him this, Rabbi Eleazar could have erred.