When the declared will of the parties does not align with their true intent, we speak of a discrepancy between the declaration of will and actual intent—commonly referred to as a defect of will. Defects of will under the OZ are special circumstances that may lead to the invalidity or annulment of a contract due to deficiencies in the expression of a party’s will. These occur when a party misunderstands or is mistaken about a specific fact or circumstance that was crucial to the contract’s conclusion. Each type of defect of will may result in the annulment of a contract if the party would have acted differently had the defect not occurred. It is essential that the defect of will be fundamental, meaning that it concerns a fact that was decisive for the contract’s conclusion rather than a minor or insignificant detail.