Legal Definitions - collateral mistake

A collateral mistake is a mistake that does not relate to the nature of the contents of an agreement, but only to some external circumstance, so that the mistake has no effect on the validity of the agreement. It is also known as an inessential mistake, nonessential mistake, or collateral mistake.

Suppose a person agrees to buy a car from another person, but mistakenly believes that the car has a sunroof. However, the car does not have a sunroof. This is a collateral mistake because the mistake does not relate to the nature of the contents of the agreement, which is the sale of the car. The mistake has no effect on the validity of the agreement, and the person is still obligated to buy the car.

Another example of a collateral mistake is when a person agrees to rent an apartment, but mistakenly believes that the apartment has a dishwasher. However, the apartment does not have a dishwasher. This mistake does not affect the validity of the rental agreement, and the person is still obligated to pay rent for the apartment.