In 750 in Lanciano, Italy, mass was celebrated by a doubting priest

A 17th-century marble inscription describes the miracle:

“A priest doubted whether the body of our Lord was really present in the host. He celebrated mass and as he pronounced the words of consecration he saw the bread turned into flesh and the wine turned into blood. Everything was visible to those present.”

It all started when a monk in the monastery of Saint Legonziano doubted that the communion bread was really Christ’s body and the consecrated blood was his blood. However, he constantly asked God to give him a stronger faith. One morning when he was celebrating mass, after saying the consecration words, doubts weighed on him more than ever. Then he saw how the host (bread) was turned into flesh and the wine into blood. After standing motionless before the miracle for a long time, he turned with a joyful and tearful face and said to those who were there:

“O blessed friends, to whom our Lord has appeared in this most holy sacrament, made himself visible to our eyes and removed my unbelief. Come and see our God who has come so close to us.”

At these words those present hastened to the altar in deep reverence.

The wonder can still be seen today in the same church in which it occurred.

The Catholic Church maintains that the bread and wine in communion are indeed transformed into the body and blood of Christ, albeit invisibly.

Several times in history this has been confirmed by miracles.

FACTS

In 1970, the Archbishop of Lanciano requested a thorough scientific investigation of the miracle that had occurred twelve hundred years earlier.

On March 4, 1971, Professor Dr. Edward Linoli a detailed report on the various studies. Here are the results:

  • The “miracle meat” is real meat consisting of tissue from the heart muscle, it is still “fresh”.
  • The “miraculous blood” is indeed blood.
  • The immune-hematological test shows that both belong to the same blood type AB, which is the most common among people in the Middle East.

A  further external survey was carried out in collaboration with the WHO. It was published in 1996 and confirmed the results above.

Source: 
https://web.archive.org/web/

A film about several Eucharistic miracles:

Read more:

Here  is a compilation of many Eucharistic* miracles performed by a young man, Carlo Acutis. Carlo died of leukemia at the age of 15 and has been beatified by the church.

http://www.miracolieucaristici.org/en/Liste/scheda_b.html

*Eucharist = The transformed bread and wine offered during Holy Communion.