An eight-person party on New Year’s Eve. Several groups of six on Saturday nights.

Danny Beck, the owner of the Pearl’s Southern Comfort restaurant in Chicago’s Edgewater neighborhood, began noticing such no-shows, often at crucial moneymaking times, at an unusual frequency in recent months. Such failed reservations leave tables open for hours, depriving servers of tips and forcing walk-in diners to be turned away.

It turned out dozens of other restaurants using Reserve, an online reservation system, were having similar problems with no-shows. And this week, as detailed in a story at Eater, the company said it had discovered the cause: An employee at OpenTable, a rival reservation system that is considered the giant of the industry, had used Reserve to place more than 300 reservations at 45 Chicago restaurants, with the intent of leaving the tables empty.

OpenTable confirmed that the employee had been fired, insisting that he or she had gone rogue and that no one else in the company knew of or directed the plan. It pledged to reimburse the restaurants for lost money and apologized, in a statement on Monday, for “this disgraceful, unsanctioned behavior.”

Some of the restaurant owners harmed by the scheme were livid.

“I just think that’s one of the lowest, dirtiest tactics I’ve ever heard,” Mr. Beck said in an interview on Monday.