Students who are hardcore gamers (i.e. most of them, if you're teaching game design) are used to seeing the marketing-speak on the back of a game box (we call this "box copy"). You've seen it before: "Over 30 levels! 300 weapons! Epic, engaging storyline! Intuitive combat system!"
Most of these students have never seen an actual game design document before. This would be the document that actually describes the details. Exactly what are the contents of each level? What are the names, damage, speed, accuracy and other effects of each weapon? What happens in the story, when exactly is each bit of story revealed to the player, how much is text and how much is voice acting, what is every last line of dialogue? How, exactly, does the combat system work and what are the controls? And so on.
It is, apparently, easy to get these mixed up. Box copy is useless if you're giving it to a programmer to implement. How does a programmer write code for "intuitive combat system" exactly? The answer is that they don't -- they kick it back to the designer until they get the details.
I'm seeing this more and more with students lately, and I'll be taking additional steps in the future to warn them of the difference between design and marketing. I wonder if other teachers see this as frequently as I am... and what, if anything, they do about it.