Contradiction with Luke 8:39
Mark 5:18 describes the man wanting to go with Jesus, while Luke 8:39 recounts Jesus telling him to return home and declare what God has done, indicating he should not accompany Jesus.
Luke 8:39: Return to thine own house, and shew how great things God hath done unto thee. And he went his way, and published throughout the whole city how great things Jesus had done unto him.
Contradiction with Matthew 8:19-22
In Matthew, Jesus discourages another person from following him immediately by urging responsibilities first, whereas in Mark 5:18, the man desires to follow Jesus but is not allowed to do so.
Matthew 8:19-22: And a certain scribe came, and said unto him, Master, I will follow thee whithersoever thou goest.
Contradiction with John 5:14
In John, after being healed, a man is later found by Jesus and told to sin no more, while in Mark 5:18, the man is not given specific instructions after his healing apart from his desire to follow Jesus.
John 5:14: Afterward Jesus findeth him in the temple, and said unto him, Behold, thou art made whole: sin no more, lest a worse thing come unto thee.
Contradiction with Mark 10:21-22
This situation presents Jesus inviting the young man to follow him, whereas, in Mark 5:18, Jesus does not permit the formerly possessed man to follow him.
Mark 10:21-22: Then Jesus beholding him loved him, and said unto him, One thing thou lackest: go thy way, sell whatsoever thou hast, and give to the poor, and thou shalt have treasure in heaven: and come, take up the cross, and follow me.