Serena Williams was eliminated in the semifinals of last year’s U.S. Open when, having lost the first set and down 5-6 in the second, she was called for a second-serve foot fault that made it match point for Belgium’s Kim Clijsters. Williams’s explosive and profanity-laced protest of the call incurred a mandatory one-point penalty that gave Clijsters the match. Although nobody defended Williams’s outburst, professional commentators and ordinary fans did debate whether a foot fault should have been called, with many maintaining that the sport’s rules should be enforced less strictly given the critical juncture in the match, and others objecting that such a practice would violate what might fairly be described as basic rule of law principles.

Although the ending to the Williams-Clijsters match was unusually dramatic, the question it raises arises frequently in the world of sports. Many fans of basketball, football and hockey, for example, routinely urge the officials to “let ‘em play” or to “swallow the whistles” in crunch time, while other observers wonder how such a practice could possibly be justified.

This essay explores whether it can be. In doing so, it draws on a wealth of popular, legal, and philosophical materials – the common sayings “no harm, no foul” and “it cost us the game”; the material breach doctrine from contract law and tort law’s “lost chance” doctrine; the mystery of objective singular probabilities and the Hartian distinction between duty-imposing and power-conferring rules; and much more. Its ambition is not merely to resolve this single – surprisingly deep and rich puzzle – but to birth a new field of sustained jurisprudential and legal-comparative study: the field of sports and law.