15 Biggest Unresolved Plot Holes In Harry Potter
13. Occlumency Lessons

In The Order of the Phoenix, Voldemort’s primary mission is to obtain a prophecy in the Department of Mysteries. Given prophecies can only be taken by those whom they are concerned with, Voldemort plants a false vision in Harry’s mind and lures him to the Hall of Prophecy.
Dumbledore (who was aware of Voldemort’s objective) tried to prevent this from happening by instructing Snape to teach Harry Occlumency – the magical defence of the mind against external penetration.
But if Dumbledore was so worried that Voldemort might try and fool Harry, why didn’t he just tell him about the Dark Lord’s mission? Wouldn’t that have been easier than forcing him to take lessons he had no interest in? At the very least, he should have warned Harry against going to the Department of Mysteries even if he didn’t want to reveal the reason behind such an instruction.