Microblog: just ask "why?"
When running an RPG, I often improvise details about the world. And, it is then often true that these details do not make any sense. For example, in a recent Stonetop session, I mentioned that a Hillfolk band the PCs were approaching weren't guarding their approach. That did not make any sense, but instead of retcon, I paused and asked, "wait, why?" The answer, after consulting the setting guide, is that there was a gouzadn (a human who has been driven to spirit-cannibalism) about! This created high drama and gave more interest to the session.