Educational research assumes that error reflections are efficient if they include the rationale behind the own error instead of just correcting the error. However, thus far there is a lack of empirical evidence regarding this aspect. Thus, we conducted a field experiment with pre-post-follow-up design and with 7th and 8th grade students (N = 174). The study was conducted during standard mathematics lessons. We compared two different error-handling strategies. Our findings indicate that students who reflected the rationales behind their errors enhanced their procedural knowledge more than students who reflected on the corresponding correct solution only. Regarding conceptual knowledge we found this effect only at the follow-up-test. The implications for theory and school instructions are discussed. [For the complete proceedings, see ED597799.]