If an online merchant make an offer by mistake on his web-site and a lot of orders are made by online customers amounting to acceptance.
For instances, the sale of vehicles at the mistakenly posted price of $1,000.00 in stead of $100,000.00. He can only rely on the doctrine of mistake to vitiate the contract. Mistake as a vitiating factor under the law of contract is provided by the common law whether the defence can be established depends on the court's find that the online customer knew that a mistake had occurred.
However, case laws are not definite as to whether the mistake must actually be known to the customer or whether it is enough if it is proven that the mistake ought to have been apparent to any reasonable man Centrovincial Estates plc v Merchant Investors Assurance Co. Ltd  Com LR 158 held that the objective test of “reasonable man should apply. However, Agip SpA v Navigazine Italia SpA  1 L’Loyd's Rep 353 held that equity would require actual knowledge for rectification.