Teachers and trainers often try to prevent learners from making errors, but recent findings (e.g., Kornell, Hays, & Bjork, 2009) have demonstrated that tests can potentiate subsequent learning even when the correct answer is difficult or impossible to generate (e.g., "What is Nate Kornell's middle name?"). In 3 experiments, we explored when and why a failed test enhances learning. We found that failed tests followed by immediate feedback produced greater retention than did a presentation-only condition. Failed tests followed by delayed feedback, by contrast, did not produce such a benefit--except when the direction of the final test was reversed (i.e., the participants were provided with the target and had to produce the original cue). Our findings suggest that generating an incorrect response to a cue both activates the semantic network associated with the cue and suppresses the correct response. These processes appear to have 2 consequences: If feedback is presented immediately, the semantic activation enhances the mapping of the cue to the correct response; if feedback is presented at a delay, the prior suppression boosts the learning of the suppressed response. (Contains 3 figures.)