Switching between tasks afforded by the same stimuli is associated with a slow-down in responding. This task switch cost is assumed to result, to some extent, from proactive interference evoked by previous processing of the current stimulus in the context of the other task, suggesting more pronounced susceptibility to proactive interference in task switch trials than in task repetition trials. To investigate whether this switch-specific enhancement of proactive interference is confined to competition from the other task of the current task context or whether it represents a more general property of task-switching performance, participants switched between a color task and a letter task featuring single-affordant stimuli (i.e., stimuli that did not allow application of the other task). Proactive interference was evoked by reversing the stimulus-response assignment in one of the tasks after the first half of the experimental session. Consistent with the notion of stronger susceptibility to proactive interference in switching conditions, this manipulation enhanced the switch cost in the task subject to the mapping reversal. Increasing task preparation by lengthening the cue-target interval from 200 ms to 800 ms did not reduce the reversal-induced enhancement of the task switch cost in reaction times (despite reducing the task switch cost overall). Our results suggest that switching between tasks—even in the absence of competition between them—increases susceptibility to proactive interference despite ample task preparation.