3. Harry Potter and the Prisoner of Azkaban
This one has been well covered too but it really is a glaringly obvious puzzling moment in the Harry Potter franchise. In Prisoner of Azkaban, Hermione is given a magical timepiece by Dumbledore to allow her to cram in some extra learning. Her and Harry later then use said time piece to go back in time and stop an unfortunate incident from ever happening. Forget all your patronus charms and polyjuice potion; this is a device which would allow our heroes to go back to wherever they want as many times as they want. Just go back to the first film and destroy Voldemort when hes still weak! In fact, just go back in time even further and kill Voldemort when he was a young lad, thus saving everyone a whole heap of trouble. It would be for the greater good. At the very least go back a little bit further on the day in question so you dont have to peg it around everywhere in such a rush.