Even if you’re not one of the reported 13 million folks who bought a brand-new iPhone 6S or 6S+ this weekend, you may want to go have a look in your phone’s settings. There’s a new feature in iOS9 that’s supposed to be convenient for consumers, but is causing overage problems and billing headaches for some users.

The offender is called Wi-Fi Assist, Quartz reports, and it’s a feature in the newest version of Apple’s mobile operating system.

Wi-Fi assist is meant to, well, assist your phone when you’re in a situation with poor or spotty wifi. That free coffee shop network not doing what you need it to? Your phone will start boosting the signal with 4G or 3G cellular data instead.

For customers with unlimited data on their plans, that’s not a problem. But for anyone who does have a metered data plan — which is tens of millions of consumers — those little “boosts” can add up to big, unexpected overages surprisingly quickly. Some users tweeted complaints of popping from their regular 1-3 GB of usage to as high as 4-7 GB.

The option is enabled by default, so if you haven’t explicitly gone into your phone’s settings to turn it off yet, it’s on right now. To disable, go to Settings –> Cellular and scroll all the way down. The Wi-Fi assist toggle is at the bottom.

You might want to turn off this new iOS 9 setting if you don’t have unlimited data [Quartz]

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