Contradiction with Malachi 4:5-6
Malachi predicts Elijah's return before the great day of the Lord, suggesting an eschatological future, while Matthew 17:11 indicates the restoration has already begun.
Malachi 4:5-6: Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD:
Contradiction with Matthew 11:14
This verse identifies John the Baptist as Elijah who was to come, which seems to fulfill Elijah's role, while Matthew 17:11 suggests Elijah's coming is still necessary to restore all things.
Matthew 11:14: And if ye will receive [it], this is Elias, which was for to come.