Apple’s iOS 9 software is already installed on well over 50% of existing devices, according to what Apple revealed in the first weekend after releasing the operating system. The new iOS version brings over a slew of improvements — though it doesn’t necessarily make older devices faster — as Apple focused mainly on performance gains rather than flashy new features.

Even so, there are plenty neat tricks available in iOS 9, including 3D Touch (only on iPhone 6s series), a smarter Siri and improved battery life. But there’s also one awesome feature that can turn out to be a nightmare for users.

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Called Wi-Fi Assist, the iOS 9 feature is enabled by default and it’s supposed to make sure that you’re getting a decent Wi-Fi experience. That means instantly switching to cellular data when Wi-Fi is spotty. For most users, that isn’t a problem, especially if they’re on large plans, or if they’re not consuming lots and lots of data by streaming video and music.

However, as Quartz explains, Wi-Fi Assist can turn out to be quite a data hog, as the device will continually try to supplement weaker Wi-Fi performance with carrier data.

The new feature can be turned off quickly — and you should do it whenever you’re roaming, or if you have dependable Wi-Fi networks at home and work. To do so, just go to Settings, then Cellular, and scroll down to the bottom to see if Wi-Fi Assist is turned on.

The blog says that the increased data consumption is not a bug, and Apple is aware of the issue — if you can call it an issue. Therefore you’ll have to fix it yourself to avoid unexpected data usage and get back to your regular monthly data consumption.