linguistic:paraphrasing
Difficulties with the rule

Although the decision of Lord Dunedin sought to bring greater clarity to the law in 1914, in practice it often proved difficult to apply. Cases continued to come before the courts challenging provisions as a penalty, and the courts continued to wrestle with the issue. In Workers Trust & Merchant Bank Ltd v Dojap Investments Ltd Lord Browne Wilkinson tried to describe the scope of the law of penalties, and noted the slightly anomalous rules in relation to forfeiture of deposits in relation to sales of land: "In general a contractual provision which requires one party in the event of his breach of contract to pay or forfeit a sum of money to the other party is unlawful as being a penalty, unless such provision can be justified as being a payment of liquidated damages being a genuine pre-estimate of the loss which the innocent party will incur by reason of the breach. One exception to this general rule is the provision for the payment of a deposit (customarily 10% of the contract price) on the sale of land..."