For decades, Philipp Lotmar has repeatedly and insistently focussed on the subject of error. In a monumental work, Lotmar set out to examine the countless relevant Roman sources in every area of law inside as well as outside the Corpus Juris Civilis, thus providing the material basis for his criticism of Karl Friedrich von Savigny's leading doctrine of errors in contract law. However, Lotmar could not complete the work he considered the crowning achievement of his examination of Romance, nor did he succeed in publishing the first, almost finished volume. The entire first volume and the introduction to the second volume, i.e. those parts of the intended opus which Lotmar left behind in a form already fit for publication, will now be presented to the public. In this way, the scientific work of a formative civil law teacher, who today is perceived primarily as the creator of modern labor law, is thus finally being made available in his proper field of research, i.e. Roman law and general private law dogmatics, almost a hundred years after his death.