In relation to the Mass, St. Gregory the Great is perhaps especially remembered by many for the Eucharistic Miracle that occurred in 595 during the Holy Sacrifice. This famous incident was related by Paul the Deacon in his 8th century biography of the holy pope, Vita Beati Gregorii Papae.
Pope Gregory was distributing Holy Communion during a Sunday Mass and noticed amongst those in line a woman who had helped make the hosts was laughing. This disturbed him greatly and so he inquired what was the cause of her unusual behaviour. The woman replied that the she could not believe how the hosts she had prepared could become the Body and Blood of Christ just by the words of consecration.
Hearing this disbelief, St. Gregory refused to give her Communion and prayed that God would enlighten her with the truth. Just after making this plea to God, the pope witnessed some consecrated Hosts (which appeared as bread) change their appearance into actual flesh and blood. Showing this miracle to the woman, she was moved to repentance for her disbelief and knelt weeping. Today, two of these miraculous Hosts can still be venerated at Andechs Abbey in Germany (with a third miraculous Host from Pope Leo IX [11th century], thus the Feast of the Three Hosts of Andechs.