The study investigated whether activation of inaccurate prior knowledge before study contributes to primary-school children's commission errors and overconfidence in these errors when learning new concepts. Findings indicate that inaccurate prior knowledge affects children's learning and calibration. The level of children's judgments of learning for recall responses for which they would not receive credit was inappropriately high after activation of inaccurate prior knowledge. Moreover, results showed that activation of inaccurate prior knowledge was not only detrimental for monitoring judgments during learning, but also for calibration accuracy after test taking. When judging the quality of their recall responses on the posttest, children were more overconfident when they had activated inaccurate prior knowledge. Also, the children often discarded concepts from further study after activation of inaccurate prior knowledge. These results suggest that in order to improve self-regulated learning, it may be important to detect inaccuracies in children's prior knowledge. (Contains 5 tables and 3 figures.)