Apple launched its latest operating system last year, but some people are still getting hit with jacked-up bills as a result of one of its new features. 

It’s called Wi-Fi Assist, and it’s designed to make your web experience faster. When you’re stuck on a shoddy Wi-Fi connection, it automatically switches your phone over to data instead. 

That’s great when your Instagram feed is finally able to refresh, but it also means that you use up more data. And if you don’t have an unlimited plan, you may go over your cap, resulting in a massive bill. 

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The kicker: The feature is automatically turned on when you update your iPhone to iOS 9. 

So if you’re running the latest operating system, you could be cruising Facebook in your living room, hooked up to your Wi-Fi, happily unaware that you’re actually sucking up your data.

Mikey Campbell, senior editor at Apple Insider, says that most people don’t see a dramatic change in their data usage because the feature kicks in only in very specific circumstances: when your data connection is faster than your Wi-Fi. 

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“Of course it depends on user habits,” he says. “For example, iPhone owners who frequent coffee shops, airports, or other establishments with open, low-quality Wi-Fi might want to turn off Wi-Fi Assist altogether.”

Sure, most cell phone companies will notify you if you’re reaching or exceeding your data limit. But if you’re connected to Wi-Fi, you’d probably think you’re safe.

That’s exactly what happened to Ashton Finegold, a San Francisco teen who ran up a $2,000 bill surfing the web from a bedroom with a spotty Wi-Fi connection, according to a CBS report.

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Attorney Bill Anderson, J.D., a partner with Cuneo, Gilbert, and LaDuca, LLP, says he’s heard from dozens of people who have been slammed with overage charges ranging from $40 to $250 and above.

Anderson has filed a complaint against Apple on behalf of two of his clients and is waiting for the company’s response. 

How to Turn Off Wi-Fi Assist

Turning Wi-Fi Assist off is easy—as long as you’re aware of the feature. Here’s how:

Wi-Fi Assist
Men's Health

Go to your iPhone settings. Click “Cellular.” Scroll all the way down to the very bottom, and if your phone is running iOS 9, you’ll see the “Wi-Fi Assist” feature. Switch the toggle to the “off” position.