During Mass the Host changed to Flesh and the wine changed to Blood and was expelled from the chalice, staining the corporal.  To this day, the Sacred Relics of the miracle are guarded near the church where this prodigy took place and numerous pilgrims go there annually to honor them. The Way of St. James passes right through O’Cebreiro. See this for yourself!!

One icy winter in 1300 a Benedictine priest was celebrating the sacred Mass in a chapel beside the church of the convent of O’Cebreiro. On that miserable day of unceasing snow and unbearably freezing wind, he thought that no one would dare show up for Mass. He was wrong. A farmer from Barxamaior by the name of Juan Santín, left the convent to attend Mass. The priest saying Mass, who did not believe in the Real Presence of Christ in the Most Holy Sacrament, despised in his heart the farmer’s sacrifice of good will. He began in this way to celebrate the Mass, and immediately after having professed the words of the consecration, the Host changed to Flesh and the wine changed to Blood, and was expelled from the chalice and stained the corporal. At that very moment, it seemed that even the head of the wooden statue of the Madonna was leaning in adoration. The people today call her the “Madonna of the Sacred Miracle”. The Lord had wanted to open the eyes of the incredulous priest who had doubted and to compensate the farmer for his great devotion. For almost two hundred years the Host-changed-to-Flesh was left on the paten until Queen Isabella learned about the miracle when she passed through O’Cebreiro while on pilgrimage in Santiago de Compostela.  The queen immediately had a precious crystal shrine custom-made to hold the miraculous Host, the chalice and the paten, which to this day, can be admired in this church. Every year on the feast days of Corpus Christi, August 15th, and September 8th, the relics are taken in procession along with the Madonna.  Among the most documented testimonials of the miracle are the bull of Pope Innocent VIII of 1487, that of Pope Alexander VI of 1496, and an account by Father Yepes.

Source: http://www.therealpresence.org/eucharst/mir/english_pdf/Ocebreiro.pdf