A local newspaper reports that a Shell station in Woodstock wound up mistakenly selling the nearly-free fuel for about two hours Sunday night.
As word spread, cars lined up and jostled to fill-up.
Woodstock Police Sgt. Dennis Leard says authorities had to use an emergency shut-off to stop the sales before the gas station closed in order to reset the pumps to regular prices.
But before that, cars were blocking traffic and lining up on the street to reach the cheap gas.
Leard says the station clerk working at the time wasn't aware of the glitch.
It wasn't clear how much gas was sold.
Woodstock is 50 miles northwest of Chicago.