If the contract is of substantial value, for example to buy or sell a house, an attorney should review the contract to ensure that nothing is overlooked. If you are ever in doubt, it is best to consult an attorney at the contract writing stage. A contract may be made for a wide variety of reasons, but to be valid, the contract must be made for a lawful purpose. Contracts to engage in illegal activity are not valid contracts and will not be upheld in court.  Examples of illegal contracts are those for the sale of illegal drugs or agreements to commit a crime. If a contract contains misrepresentations (whether or not there was a specific intent to defraud), then the contract will be invalid.  Double-check all information in the contract and be as specific and clear as possible to avoid possible accusations of misrepresentation or fraud. For a contract to be valid, it must have these three basic elements: a specific offer,  acceptance of the terms of the offer, and consideration, which is the agreed-upon exchange of goods or services.   A valid offer must be sufficiently definite. It must be clear, unequivocal, and direct. Make sure you have all the parts of a contract. Counter-offers are sometimes attached to a contract. However, the introduction of a counter-offer changes the contract. With most contracts, a counter-offer replaces the original offer and is treated as though it is a new offer.  There must be acceptance or, in the absence of formal acceptance, performance. Acceptance must be made in the mode or method established by the offeror, and it must be made before the deadline in the contract. Although silence will not qualify as acceptance, certain actions may. For example, if a person sends in an order for goods, and the seller responds by sending the goods, then the seller has manifested acceptance of the offer.  A contract must contain consideration: mutual promises to do something or to refrain from something that a party has a legal right to do. Without this mutual promise, there is no valid consideration and the contract is illusory.

Summary:
Hire an attorney to review the contract. Determine whether the subject matter of the contract is lawful. Search for erroneous information. Identify an offer, acceptance, and consideration.