A fat-finger error is a keyboard input error or mouse misclick in the financial markets such as the stock market or foreign exchange market whereby an order to buy or sell is placed of far greater size than intended, for the wrong stock or contract, at the wrong price, or with any number of other input errors.
Automated systems within trading houses may catch fat-finger errors before they reach the market or such orders may be cancelled before they can be fulfilled. The larger the order, the more likely it is to be cancelled, as it may be an order larger than the amount of stock available in the market.
Fat-finger errors are a product of the electronic processing of orders which requires details to be input using keyboards. Before trading was computerised, erroneous orders were known as "out-trades" which could be cancelled before proceeding. Erroneous orders placed using computers may be harder or impossible to cancel.