Contradiction with Malachi 4:5
Mark 9:11 discusses the prophecy of Elijah coming first, which in Malachi 4:5 is interpreted as a future event, but Jesus indicates that Elijah has already come in the form of John the Baptist.
Malachi 4:5: Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD:
Contradiction with Matthew 17:12-13
Mark 9:11 raises the question about Elijah's coming, while in Matthew 17:12-13, Jesus explains that Elijah has already come, causing confusion among the disciples.
Matthew 17:12-13: But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them.