Watch a video while it downloads in iTunes 10
As of iTunes 12, you cannot watch a movie while it is downloading. This is a pretty recent change, but has been proven and is 100% reproducible. You have to wait for the movie or TV show to finish downloading. (Trying to play before it finishes downloading shows a black screen, until download is finished -- then play begins as usual). This is a real shame and makes no sense, given that an Apple TV can actually play while streaming... but, guess that's different software. (Netflix for the win!)