Usually, once a contract has gone through the process of offer and acceptance, both parties agree to be bound by its terms. However, even after a contract has been signed, there may be circumstances where it is found to be invalid, or it becomes invalid because of an action by one side or the other.
Chapter four of the Principles of European Contract Law describes why a contract may become invalid, either from the start or at some point after it was signed. For example, factual or legal errors may make a contract invalid, as can fraudulent conduct during the negotiations.
After a contract is signed, it can become invalid if one side or the other fails to deliver what is agreed.