Florida gas customers have something to worry about other than paying above the national average per gallon: malfunctioning pumps that could be siphoning even more of their hard-earned money.

A recent study conducted by South Florida Sun-Sentinel found as many as 34% of the gas stations in the state have at least one pump that is inaccurately dispensing the wrong amounts of gas. In most cases, the error has worked in customers’ favor, giving them a bit more gas for their money. In almost as many cases, however, the opposite has been happening, giving drivers less gas than they’re paying for.

The Sun-Sentinel analyzed state inspection reports from 2004 to 2006. The analysis found 580 of more than 2,500 stations in South Florida had at least one pump dispensing more gas than customers paid to purchase, while 477 provided less fuel than they should.

“If you go to the grocery store and buy a gallon of milk, you expect a gallon of milk,” said Jason Toews, co-founder of Gasbuddy.com, a consumer advocacy site that tracks gas prices, told the newspaper. “The same goes for gasoline.”

In some rare cases, gasoline retailers are actually augmenting the pumps themselves to pull in a few extra bucks. By and large, however, most of these erroneous pumpings are accidental, according Jim Smith, president of the Florida Petroleum Marketers and Convenience Store Association.

“We have a bad reputation as it is right now because of the gas prices,” Smith told the Sun-Sentinel. “You don’t want the consumer thinking they’ve been cheated every time they buy gas.”

It’s unclear if the Florida is the only state that has such a high percentage of this occurrence, but the newspaper reported that in recent years, the state’s results have mirrored that of the nations.