Cursed Child Committed a Crime Against Prisoner of Azkaban
In Harry Potter and the Prisoner of Azkaban, we are introduced to the concept of a Time-Turner, a magical device that allows the user to go back in time.
Your first thought may have been, "why doesn't Harry go back in time and save his parents?
The reason quickly becomes clear: in the Harry Potter universe, time travel is a closed loop. That means any actions taken while in the past have already happened and cannot be changed. You may travel to the past, but it's impossible to actually change the past.
On their first trip through the timeline, a stone smashes through Hagrid's window and Harry is saved by a mysterious figure with a stag Patronus. Those things happen again after they've gone back in time – it's just that now Harry and Hermione are the cause of these events, rather than the witnesses.
The concept of a closed loop is often used in science fiction to maintain consistency in time travel stories and avoid creating paradoxes, and Harry Potter is no exception. This rule explains why Harry can't go back and save his parents, and why nobody can go back and simply kill Tom Riddle when he was a baby. The past cannot be changed, only understood from a different angle.
However, in Harry Potter and the Cursed Child, a play set nineteen years after the events of the original series, the rules of time travel are changed. When Albus Potter and Scorpius Malfoy travel back in time to save Cedric Diggory on the final night of the Triwizard Tournament, they succeed in radically changing the past – altering the timeline completely and causing Voldemort to return.
(Also Cedric grows up to be a Death Eater and kills Neville Longbottom? Hot take: Cursed Child is dumb.)
The established rules of time travel in Prisoner of Azkaban were essential to the narrative and created tension as well as high stakes for the time travelers – having to work in a closed loop means that the kids can't just keep going back in time over and over to create the perfect future.
When people die, they stay dead. When harm has been done, it can't be undone. The departure from these rules in the later work undermines the logic of the Harry Potter universe and makes it completely unbelievable that Voldemort and Dumbledore wouldn't simply have been in a Time War for decades, returning to the past again and again to find different outcomes to their stories.
While time travel was an exciting plot device in Prisoner of Azkaban, JK Rowling shot herself in the foot here – and did her own books a huge disservice.