Contradiction with Matthew 8:28
This verse refers to two men possessed by demons, whereas Mark 5:2 mentions only one man.
Matthew 8:28: 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.
Contradiction with Luke 8:27
Similar to Matthew, this verse refers to a single man but describes him as having been possessed for a long time, which might contradict the immediate context of possession in Mark 5:2.
Luke 8:27: 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.