Transitioning from a centralized to a distributed decision-making strategy can create vulnerability to adversarial manipulation. We study the potential for adversarial manipulation in a class of graphical coordination games where the adversary can pose as a friendly agent in the game, thereby directly influencing the decision-making rules of a subset of agents. The adversary's influence can cascade throughout the system, indirectly influencing other agents' behavior and significantly impacting the emergent collective behavior. The main results in this paper focus on characterizing conditions by which the adversary's local influence can dramatically impact the emergent global behavior, e.g., destabilize efficient equilibria.