According to this Reuters report, the estate of Paul Allen, author of the 1987 novel The Adventures of Willy the Wizard -- No 1 Livid Land, are claiming that J.K. Rowling copied "substantial parts" of Allen's book when she wrote Harry Potter and the Goblet of Fire.
Allen's estate alleges that "the plot of Harry Potter and the Goblet of Fire copied elements of the plot of Willy the Wizard, including a wizard contest, and that the Potter series borrowed the idea of wizards traveling on trains.
"Both Willy and Harry are required to work out the exact nature of the main task of the contest which they both achieve in a bathroom assisted by clues from helpers, in order to discover how to rescue human hostages imprisoned by a community of half-human, half-animal fantasy creatures," the estate statement said.
Let me be the first to call this plagiarism claim what it is: Bull crap! (Yes, I'm being polite by using the euphemism.) Having wizards in different books both riding on trains is not plagiarism--that's next, yelling plagiarism if two authors both write about characters riding in cars? And having Willy and Harry both work out their task assisted by clues from helpers--My God, that sounds like the plot of half the fantasy books in existence. That must mean every fantasy author out there is guilty of plagiarism.
Seriously, this is an attempt by the estate of an author no one has heard of to profit from a book no one has read. Google The Adventures of Willy the Wizard and all you'll find is a report about this lawsuit. According to a Google Book search, Allen's book is all of 36 pages long and was likely published (according to the Writer Beware blog) by a vanity publisher. Even if these charges rose to the level of plagiarism--which they don't--I rather doubt Rowling could have found a copy of Allen's book to read.
So like I said earlier: Bull crap!