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 you have signed a contract, there may be circumstances where it is invalid. Contracts can also become 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. For example, factual or legal errors may make a contract invalid, as can fraudulent conduct during the negotiations.
After you sign a contract, it can become invalid if one side or the other fails to deliver what they have agreed.