A good RPG campaign usually takes on a life of its own. The longer you play, the more the characters, the places, and the events of a game overshadow the rules which you use for the game. Ironically, it’s this shift in importance away from mechanics which can sometimes reveal that the mechanics you’ve been using aren’t going to work for an important part of your ongoing game. In another situation, your campaign has taken a dramatic, albeit temporary, turn. Your grizzled heroes find themselves masquerading as schoolteachers, or your starship crew finds a rip in the space-time continuum, or your cyberpunks have to chase a villain into a virtual reality game. Whether it’s a mid-story diversion or a permanent change, sometimes you’re going to want to jump systems.