By Brendan I. Koerner, Wired

In early June 2014, accountants at the Lumiere Place Casino in St. Louis noticed that several of their slot machines had—just for a couple of days—gone haywire. The government-approved software that powers such machines gives the house a fixed mathematical edge, so that casinos can be certain of how much they’ll earn over the long haul—say, 7.129 cents for every dollar played.

But on June 2 and 3, a number of Lumiere’s machines had spit out far more money than they’d consumed, despite not awarding any major jackpots, an aberration known in industry parlance as a negative hold. Since code isn’t prone to sudden fits of madness, the only plausible explanation was that someone was cheating.

Casino security pulled up the surveillance tapes and eventually spotted the culprit, a black-haired man in his thirties who wore a Polo zip-up and carried a square brown purse. Unlike most slots cheats, he didn’t appear to tinker with any of the machines he targeted, all of which were older models manufactured by Aristocrat Leisure of Australia. Instead he’d simply play, pushing the buttons on a game like Star Drifter or Pelican Pete while furtively holding his iPhone close to the screen.

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