A contract can only be contested if it is defective in some way. The following defects in particular can affect a contract:
Article 21 of the Swiss Code of Obligations (CO) regulates the case of unfair advantage. Where there is a clear discrepancy between performance and consideration in a contract that has been concluded through one party's exploitation of the other's position of need, inexperience or thoughtlessness, the party who is at a disadvantage may declare within one year that they will not honour the contract and may demand restitution of any performance already made.
Articles 23 and 24 CO regulate the case of contracts concluded in error. They provide that a party who makes a serious mistake as to the circumstances when entering into a contract is not bound by that contract. The error must be substantial, i.e. so serious that if the party in error had known the true situation, they would not have signed the contract. It is advisable to inform the other party in writing that the signed contract is considered invalid on the grounds of a substantial error. If a party has been induced to enter into a contract by a fraudulent misrepresentation made by the other party, the contract does not bind the deceived party (Art. 28 CO). Fraudulent misrepresentation arises where a party to a contract lies about or fails to disclose a certain fact, such as falsely claiming that goods are of a certain quality. It is advisable to inform the other party in writing that the contract is considered void on the grounds of fraudulent misrepresentation.
It should also be noted that exercising a right of withdrawal does not prevent someone from renegotiating the contract. For further information on this subject, see the FAQs.