I had this happen once on a freebie. It was a pre-order for a book that is yet to be released, and it may have been a pricing error. It was listed as free, but when I clicked the pre-order button, it behaved as though I was purchasing (choose payment screen came up). The book looked good, and I decided to take my chances. As of today, it's still showing up in my digital orders with a price of $0.00. The book is set to release Feb. 27. I'll let you know if anything changes. YMMV, of course, but I thought I'd share my experience.