Educational research indicates that error reflection, especially reflection on incorrect examples, has a positive effect on knowledge acquisition. The benefit of error reflections might be explained by the extended knowledge of incorrect strategies and concepts (negative knowledge) which fosters the learning of new content. In a field experiment with a pre-post-design we taught fractions to N = 195 6th grade students and compared two conditions that encouraged reflection on either incorrect or correct examples. We found that incorrect examples supported students' negative knowledge more than correct examples. However, regarding the knowledge of fractions, only advanced students could benefit from incorrect examples; students with low prior knowledge learned more from correct examples. Even though negative knowledge showed a partial mediation effect for knowledge acquisition, it did not mediate the effect of error reflections on the acquisition of knowledge of fractions. The implications for school instruction are discussed.