Emergent gameplay doesn't happen as often as designers would hope for. Uh, a video game fundamentally is a number of systems interacting with each other that the players can the go in and mess with and tinker with and then experience a game. Emergent gameplay is when those systems interact in ways the designer didn't anticipate. But the players find these new ways. It doesn't happen as often as it could, should but we continue to try as creatives cause it's one of the things that's seen as the holy grail of game design.