I agree with Peter on this one. Which may be surprising to people who know me as I have shelves full of books. The problem is when people read the books and watch the videos before training and don't realize that actual training will probably be nothing like what they've seen/read.
I had one gentleman who watched an entire class. When I asked if he had questions he told me he was a "student of ki" (he pronounced it kai). He'd read books and articles and was quite knowledgeable. He asked if we did any "kai" training and I said "all the time, training the techniques is great ki training". He left after two classes. It didn't fit his idea of what learning aikido was. There were no flashes of golden light, or people being tossed with a finger flick. Just students struggling to get better, sweating and falling.