We study a collaborative location-based game in which groups of 'lions' hunt together on a virtual savannah that is overlaid on an open playing field. The game implements a straight-forward approach to location-based triggering in which players must be in the same spatial locale in order to share information and act together. Comparison of video recordings of physical play with system recordings of game events reveals subtle and complex interactions between highly dynamic player behavior and the underlying technology. While players exhibit a fluid approach to group formation, the system embodies a more rigid view, leading to difficulties with sharing context and coordinating actions, most notably when groups of players span virtual locale boundaries or initiate actions while on the move. We propose techniques for extending locales to support more flexible grouping and also discuss the broader implications of our findings for location-based applications in general.