The doctrine of mistake relates to mistakes which are in existence at the time of the formaton of the contract.
The general rule is that it two parties are in agreement the courts will not help a party to escape his obligations by claiming that he made a mistake at the time he entered into it.
However, if a court finds that there has been a mistake, depending on the type of mistake and its seriousness, the contract will be rendered either void or voidable.
Three types of mistake
Common mistake: both parties make the same mistake; each is mistaken about the same fundamental fact.
Mutual mistake: both parties are mistaken but they have made different mistakes; they are at cross purposes.
Unilateral mistake: only one of the parties is mistaken and the other party either knows of his mistake or is taken to know of it. Often, one party seeks to take advantage of the other party's mistake.