Generally, when participants have different understandings of how the game-world works that come out in play. For example, as a player you might think that your tough fighter can kill a charging boar with his sword with little fear of injury, while your GM thinks that a boar can easily ignore any sword swing and will break both his legs. You say "I crouch and prepare to meet its rush" and get severely mauled. A term from RGFA discussion.