Contradiction with John 20:12
John describes two angels sitting where Jesus' body had been, in contrast to the one young man (angel) in Mark 16:5.
John 20:12: And seeth two angels in white sitting, the one at the head, and the other at the feet, where the body of Jesus had lain.
Contradiction with Matthew 28:2-3
Matthew mentions one angel who descends from heaven and rolls away the stone, differing from Mark's young man inside the tomb.
Matthew 28:2-3: And, behold, there was a great earthquake: for the angel of the Lord descended from heaven, and came and rolled back the stone from the door, and sat upon it. [was: or, had been]
Contradiction with Luke 24:4
Luke mentions two men standing by in shining garments, differing from the single young man in Mark 16:5.
Luke 24:4: And it came to pass, as they were much perplexed thereabout, behold, two men stood by them in shining garments: