To celebrate the arrival of Harry Potter
First the runnerups:
The Bible: Where did Cain's wife come from?
Star Wars: A New Hope: Darth Vadar can sense the presence of his son Luke from across the galaxy (ep vi), but when his daughter Leah is standing directly in front of his face (ep iv) he feels nothing?
Harry Potter and the Chamber of Secrets: The entrance to the Chamber of Secrets, built and hidden by Salazar Slytherin, founder of Hogwarts, approximately one thousand years ago, is accessed through indoor plumbing
And the winner, a plot hole big enough to fit the egos of me, Godol Hador, and at least 6 of your least favorite ultra-Orthodox community leaders, is...
Harry Potter and the Goblet of Fire
Voldermort's snuck a secret agent on to the grounds of Hogwarts, for the purpose of kidnapping Harry, but instead of capturing Harry straight off, the secret agent waits a full year. For what? Why wasn't Harry snatched on the first night of the term?