The article addresses methodological problems, which exist in Russian civil doctrine, regarding the interpretation of the idea of inadmissible contradictory behavior and of Art. 166(5) and 432(3) of the Russian Civil Code on irrelevance of the statement of invalidity of a juristic act. It shows the continental history and the romanist roots of the said idea, inefficiency of correlation of the corresponding provisions of the Russian Civil Code with the estoppel in common law legal systems, and also substantiates the view that this idea, not being a principle of civil law, has always been and remains nothing more than a legal maxim (aphorism) contributing to the interpretation of the requirement of good faith in individual cases.