Concept mapping is discussed as a means to promote meaningful learning and in particular progress in reading comprehension skills. Its increasing implementation necessitates the acquisition of adequate knowledge about frequent errors in order to make available an effective introduction to the new learning method. To analyse causes of errors, 283 A-level sixth graders produced concept maps about two differently complex subject matter lessons, we implemented in a pre-lesson. We defined six types of errors and analysed the distribution and contingency tables in both subject matters. Students in general produced more complex concept maps in the context of the easier subject matter (A) than that of the difficult content (B). Whereas in the former errors simply indicated knowledge gaps, in the latter they often reflected technical misconceptions. The occurrence of a content-dependent technical error in (B) pointed to a cognitive overload, since the more difficult content is hypothesised to cause higher intrinsic load. From this following, concept mapping could provoke an instructional enrichment by additionally revealing specific knowledge gaps. (Contains 4 tables and 4 figures.)