I considered deceptively complex or deceptively simple (depending on how you interpret this previous post) as a way of expressing this. But, I would like to avoid the potential for misunderstanding and just use an idiom or expression that is unmistakable.
For example, John gravitated towards enrolling in the college course Children's Literature assuming it to be an easy A, but he was deceived. The course is in fact ______________________. (INSERT IDIOM OR EXPRESSION IN BLANK)