A recent New York Times Magazine article (“Why Flunking Exams is Actually a Good Thing,” Sep. 4, 2014) describes a fascinating line of research by Robert Bjork, Nicolas Soderstrom, and their colleagues at UCLA. The gist is this: Taking a test on material that you are not familiar with turns out to help you learn more effectively than if you had spent the same amount of time studying the material in a conventional way. (In the studies, subjects either took a test and then studied, or just studied for the equivalent amount of time, then took a different test.)
If that doesn’t seem totally surprising, consider that this effect holds even–in fact, especially–when you know nothing about the material on the test–that is, when you can do no better than random chance in answering questions about it.