The subject matter of this article is a discussion of an error as a defect in declaration of intent and its impact. For this reason, the Author will examine concepts specified in the article 84 of Polish Civil Code and present their essence. Further, the Author enumerate conditions of an error and discuss them. For this reason it will be compere the legal terms - „legal act” and „declaration of intent”. In order to discuss this matter it is necessary to explain the difference between error – in narrow meaning – and mistake. The article discuss creating competencies which enables evading defective declaration of intent made under an error, i.e. defective declaration meeting conditions which do not render invalidity of contract ex lege. For this it is necessary to act erransa. In article it will be also indicated which are the differences of opinion in the scope of evaluation of materiality of errors, i.e. subjective and objective. Considerations shall refer to the interpretation provisions of declaration of intent. At the end it shall be made the reference to error as to the motivational sphere – in particular error as to the motives. Aforementioned conditions will be discussed and compared in doctrine and jurisdiction. The article will finish with de lege ferenda conclusions.