HOUSTON, Texas (KTRK) -- With four Jack in the Box customers in the Houston area reportedly being charged a gratuity with their meal, the fast-food chain says an employee error was the cause of the mistake.A spokesman for Jack in the Box said a setting on registers "inadvertently activated" a prompt that allows a gratuity to be entered.Usually, an employee will enter "0" when prompted, but one worker mistakenly entered an amount."Apparently at least one employee didn't know what to do since the prompt was an aberration, and we were contact by four guests who were mistakenly charged a gratuity," the spokesperson told Eyewitness News.The restaurant issued a statement on the mistake after a customer posted his receipt to Facebook that included the error. He had ordered three egg rolls and a small-size order of curly fries that should only cost him $6.58.However, the receipt showed a gratuity of the same amount, adding to a total of $13.16.When he took the receipt back to the store, the customer claimed a manager accused him of being a liar.Jack in the Box said it was working with the guests to resolve the error. It added that anyone who believes he or she was inappropriately charged a gratuity can contact its guest relations department through its website.