Mistake is an erroneous assumption relating to facts or to law existing when the contract was concluded.
1. Mistake of fact and mistake of law
This Article equates a mistake relating to facts with a mistake relating to law. Identical legal treatment of the two types of mistake seems justified in view of the increasing complexity of modern legal systems. For cross-border trade the difficulties caused by this complexity are exacerbated by the fact that an individual transaction may be affected by foreign and therefore unfamiliar legal systems.
2. Decisive time
This Article indicates that a mistake must involve an erroneous assumption relating to the factual or legal circumstances that exist at the time of the conclusion of the contract.
The purpose of fixing this time element is to distinguish cases where the rules on mistake with their particular remedies apply from those relating to non-performance. Indeed, a typical case of mistake may, depending on the point of view taken, often just as well be seen as one involving an obstacle which prevents or impedes the performance of the contract. If a party has entered into a contract under a misconception as to the factual or legal context and therefore misjudged its prospects under that contract, the rules on mistake will apply. If, on the other hand, a party has a correct understanding of the surrounding circumstances but makes an error of judgment as to its prospects under the contract, and later refuses to perform, then the case is one of non-performance rather than mistake.