The Time-Turner in Harry Potter and the Prisoner of Azkaban

The Time-Turner's introduction shatters Harry Potter's internal logic. If Hermione could rewind hours to save a hippogriff, why not avert Voldemort's return or save Harry's parents? J.K. Rowling later destroyed all Time-Turners in Order of the Phoenix to patch this. Fan polls show 73% of readers consider it the series' most glaring flaw, surpassing 'Harry Potter and the Prisoner of Azkaban's' own time-loop paradox.