In the accounts of Jesus healing the demoniacs at Gadara (Matthew 8:28-34, Mark 5:1-20, Luke 8:26-39), Matthew mentions two demoniacs, whereas both Mark and Luke mention one:

Matthew 8:28

28 And when he was come to the other side into the country of the Gergesenes, there met him two possessed with devils, coming out of the tombs, exceeding fierce, so that no man might pass by that way.

Mark 5:1-2

1 And they came over unto the other side of the sea, into the country of the Gadarenes.
2 And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit,

Luke 8:26-27

26 And they arrived at the country of the Gadarenes, which is over against Galilee.
27 And when he went forth to land, there met him out of the city a certain man, which had devils long time, and ware no clothes, neither abode in any house, but in the tombs.

The simple solution to this alleged contradiction is to recognize that omitting a mention of something is not the same as asserting it did not happen. Nowhere in Mark or Luke does it say only one, and not two people were healed. If there were two people, it's also true that there was one, and Mark and Luke chose to focus on one in particular.

There are many such instances of the Gospel accounts including differing details that supplement, rather than contradict each other. Christians are well aware of this, and understand that no single account is intended to be utterly exhaustive. In fact, such a thing is explicitly denied in the Gospel of John (John 21:25).