Needless to say, spoilers follow for Harry Potter and the Cursed Child. #KeepTheSecrets
In the climactic moments of Cursed Child, Albus Potter and Scorpius Malfoy are stuck at Godric's Hollow in 1981, while Delphi plans to interfere with Voldemort's attempt to kill young Harry Potter and his family, and thereby bring about a new age of evil. Albus and Scorpius manage to send a message forward in time to adult Harry and co., who duly appear, moments later, in 1981 to help defeat Delphi.
A brilliant plan – except for one thing: How do we ‘know’ that Harry will be around in the future to receive the message?
Albus' and Scorpius' plan to seek help from future Harry relies on Harry surviving to the future (or a recognisable version thereof) to be able to receive the message and in turn travel to the past and help make sure he survives. Harry must live to the future in order to make sure that Harry lives in the past: a causal loop!
To put it another way, suppose that Delphi were to be successful in interfering in 1981 and bringing about Voldemort's rise to power. In this case, Harry would not live to receive Albus' and Scorpius' message from the past (or even if he did live, would be in a completely different situation and similarly unable to receive or act on the message). Harry and co. would not show up in 1981, and Delphi, accordingly, would succeed. Both outcomes – Delphi succeeds, and Delphi fails – lead to causal loops, but both are equally self-consistent.
So when Albus and Scorpius send the message to the future and Harry shows up, it would seem that the universe has already decided that Delphi will fail – for if Delphi succeeded, Harry would not have shown up. As soon as Harry successfully arrived in 1981, Delphi was predestined to fail.
For a play that begs its audience to ‘keep the secrets’, it sure is ironic that its ending is spoiled by its own plot! Perhaps this is why the Ministry is so hell-bent on destroying all the Time Turners…