The Miracle at Lanciano

The Miracle at Lanciano is widely considered the first and greatest Eucharistic miracle of the Catholic Church. This miracle occurred in reaction to an unnamed Basilian monk's doubt about the Real Presence of of Christ in the Eucharist. The Church teaches that the Real Presence of Jesus Christ is present in the eucharist, in the sense that the Eucharist's bread and wine become the body and blood of Christ. The Miracle of Lanciano took place in the 8th century A.D. in a small church called St. Legontian, on the Adriatic Sea in Italy. During the consecration of the host, the bread became live flesh from a human heart and the blood became real blood from the same body. The  blood type from the Miracle of Lanciano matches the blood type found on the Shroud of Turin, indicating that the Eucharist truly transformed into the body and blood of Christ.

Though it's very self-evident that the Miracle at Lanciano is connected to the Real Presence doctrine, it's equally obvious that an actual transformation (like the one that happened at Lanciano) does not occur at every celebration of the Eucharist. The actual transformation of the host to flesh and blood was a miracle to confirm believers' faith in the Real Presence doctrine of the Church. The difference between ordinary Eucharist and what occurred at Lanciano is that the ordinary transformation of the host is beyond sensory experience; Christ merely made it perceptible in one instance at Lanciano. However, just because we cannot see or feel this transformation does not mean that it doesn't happen. For example, we don't see our brains fire their neurons or see the stars burn up hydrogen as fuel, but we take it on faith that these phenomena  do occur.

http://www.catholic.com/magazine/articles/why-lanciano-matters