R. Hillel, says (Sanhedrin 99A) that there will be no Messiah for Israel, "since they have already enjoyed him during the reign of Hezekia."
Rabbi Yosef replied, "May Rabbi Hillel's Master forgive him," suggesting that Rabbi Hillel had made a terrible mistake.
However...
1 - That "terrible mistake" is enshrined on the pages of the Talmud. If it was so terrible, why did Rabina and Rav Ashi include it? Did every mistake get recorded for posterity?
2 - Rabbi Hilel was an amorah (some say he was tanna) Theologically, do we believe it's possible for an amorah or tannah to make a "terrible mistake?" Wasn't Nosson Slifkin recently raked over the coals for this?
3 - How could Rabbi Yosef be certain Rabbi Hillel was wrong. This isn't a question of theology, or law, but a question of fact: Was Hezekia the moshiach, or not? This is an empirical question; it can't be solved by a Sanhedrin.