A strategy is uniquely justifiable (UJ) if, at each decision point, the prescribed action is the unique best response to any contingency the player may consider. We define UJS mechanisms as mechanisms in which the dominant strategy is also uniquely justifiable, so that boundedly rational players do not have to perform contingent reasoning. By contrast, the leading alternative, Obviously Strategy Proof (OSP) mechanisms, require players to contingently reason about their own future actions. The presence or absence of uniquely justifiable strategies neatly organizes existing evidence of adherence — or non-adherence — to (obviously) dominant strategies across mechanisms, from binary allocation problems to matching problems. We characterize the set of UJS mechanisms for binary allocation problems to show that they resemble a generalized Multiple Price List. To compare OSP and UJS mechanisms in a setting that avoids confounds, we conduct an experiment using novel elicitation mechanisms of each type. We identify a tradeoff: OSP mechanisms fail because players fail to forecast their future actions, while UJS mechanisms allow other forms of boundedly rational mistakes.