This work presents the civil law consequences of undeclared work contracts. It primarily focuses on the agreements involving undeclared payments that pertain to tax evasion or violations of tax regulations. A chronological overview of the Federal Court of Justice (BGH) rulings regarding contractual claims and potential restitution claims is provided, covering the period from 1990 to 2014. The cases adjudicated by the BGH are subsequently subjected to a literature-based evaluation as well as an original assessment. Additionally, within the context of undeclared payment agreements, considerations regarding potential claims arising from unauthorized management and culpa in contrahendo are discussed. Furthermore, a brief overview of the BGH's rulings concerning the category of gift circles is provided, highlighting the parallels and similarities.