Reaction time is typically longer on trials on which the task changes. This switch cost is reduced by the opportunity to prepare for the change before the stimulus onset, but there remains a residual cost that resists reduction by further opportunity for preparation. De Jong (2000) proposed a model for evaluating the contribution to the residual cost of (1) failure to achieve endogenous task-set reconfiguration on a proportion of trials, and (2) limitations to the completeness of reconfiguration attainable by endogenous means. We report good fits of the model to the data from one previous and one new task-switching experiment, suggesting that the residual switch cost may indeed be attributable to a probabilistic failure to complete advance preparation. But strong incentives for preparation only marginally increased the estimated preparation probability, suggesting some intrinsic limitation to the ability to achieve endogenous preparation for a task switch on every trial.