It depends upon the circumstances. If it is a matter of impracticality; reformation of the contract can be accomplished with minimum loss to either party. If performance is impossible, then recission is probably the only alternative. Recission, however, is not always without cost. If the other party to the contract is unwilling to accept recission; he/she may seek to enforce the contract by legal action.
Example: A signs a contract to purchase an agreed upon piece of real estate. A soon learns that he will be unable to meet the full purchase price, but may be able to pay a lesser price. The seller may elect to accept the lesser price and thereby save the deal. However, if A simply seeks to rescind the contract without consideration, he has defaulted, and the seller may sue for damages, or, in the case of real estate, sue for specific performanc.