A few of them have taken me up on the offer, but mostly with off the cuff kinds of questions that most people who've read the books in the class would also remember. Until yesterday when a student in my last class came up to me after obviously putting in at least a little more thought as to what might stump me:
Student: "You know how you said that you'd be able to answer any Harry Potter question? Well, I think I have one that you won't know the answer to."
Me: "Go ahead."
Student: "What is Harry's vault number in the bank?"
Me: "713."
Student: "Dang it."