From the Lives and Sayings of the Desert Fathers:
A certain anchorite, primarily out of ignorance, did not want to accept that the Holy Bread which is received in the Holy Eucharist is the true Body of Christ. When the elders of the Skete learned this, they called him to teach him the correct teaching of the Church regarding the Holy Eucharist. He however insisted on his delusion, and the fathers left him, but prayed for him that God would enlighten him with the truth.
One Sunday the anchorite attended the Divine Liturgy from within the Holy Altar of the Church of the Skete. At the moment when the priest took in his hands the offering bread for the Proskomide, the deluded monk was stunned to see an infant laying on top of the holy altar table. And when the priest began to dissect the Bread, an angel appeared holding a knife in his hands.
As the priest was dissecting the Bread, at the same time the angel dismembered the infant and poured His blood in the Holy Chalice. The anchorite was shocked. A little while later, when he went to commune, something happened to him even more fearful. He looked into the Holy Chalice and saw human flesh in human blood.
Upon seeing this, the deluded monk wept and confessed his delusion, praying to the Lord to cover the Holy Gifts with His grace so that he may take courage and approach to receive. He looked into the Holy Chalice and this time saw bread and wine, and he thanked God after receiving the Holy Gifts.