In was the night before Pentecost in a Benedictine abbey in the city of Favernay, France. The monks erected a provisional altar for adoration of the Eucharist. The monstrance being used was large and they placed two Hosts inside. When their time of prayer was over, they left the monstrance exposed on the altar. The next morning, they returned to find the church full of smoke. As the smoke began to clear, they frantically looked for the altar, hoping to find the monstrance still on it. The were despondent upon discovering that the altar had been totally consumed. They saw the monstrance, however. It was there, hovering in the air. A multitude gathered to view the phenomenon.
They quickly erected a new altar and celebrated Mass. At the time for the elevation of the Host, the monstrance descended and rested upon the altar.
One of the hosts was donated to the Church of Dole, and it was destroyed during the French Revolution. The other Host is still available for viewing to this day.
Source: Source: “The Eucharistic Miracles of the World,” a Vatican international exhibition, as reported by The Real Presence Eucharistic Adoration Association, http://www.therealpresence.org/eucharst/mir/english_pdf/Faverney.pdf.