It's hard to but you actually can. The crackers can easily burn if you're not careful but the biggest ingredient that is going to overcook is going to be the marshmallow fluff or marshmallows.  The more that you cook a marshmallow and overcook it, the harder of a texture it's going to be. This doesn't make it inedible, it just makes it hard and chewy as it cools.  Some people actually prefer their marshmallow or marshmallow fluff a bit burnt and toasted which is totally up to preference and taste.  You'll know after you make the first one.