Rabbi Yaakov ben Meir; a grandson of RASHI; one of the greatest of the Baalei Tosafot, He is supposed to have said, "I could have written RASHI's commentary on the Talmud, but not his commentary on the Bible. He had two famous Torah disagreements with RASHI: one involving the placement of the "Mezuzah," the other involving the "Parshiyot" contained within the "Tefilin."
Moshe Rabbeinu himself is supposed to have come to him in a dream and informed him that RASHI's opinion was indeed correct. In his response, which indicated his genius and his self-confidence, Rabbeinu Tam ("Tam" means perfect) is said to have responded, "Moshe, our Teacher, here you have made a mistake."
Sources: Orthodox Union