Nnnot really. The Time-Turner, certainly, but that doesn't make the story uninstantiable. Making a logical impossibility a basic plot premise... sounds like quite an interesting challenge, but that would be a different story.
A spell that lets you get a number of objects that is an integer such that it's larger than some other integer but smaller than it's successor, used to hide something.
Here's the new thread for posting quotes, with the usual rules: