Apple debuted its iOS 9 operating system update last week, which improved on a few things here and there: a redesigned native news aggregator, a beefed up Notes app, a battery life-saving "low power mode," among other tweaks.
What you might not have noticed, however, was the addition of a new default setting for your iPhone: "Wi-Fi Assist." Letting it alone could cost you.
The idea behind the feature is well-intentioned: Boost smartphone speeds. Unfortunately, the setting does so at the expense of data usage. Apple's (aapl) software engineers appear to have prioritized one over the other.
If Wi-Fi Assist is left activated, your iPhone will automatically use cellular data whenever it finds that a Wi-Fi router signal is weak. This results in faster network connections for your phone, but it will eat up the data allotted by your cell phone plan. If you don't have an unlimited data plan, this could be a big problem when your next phone bill arrives. That's right, extra fees.
To avoid possible data overage charges, simply deactivate the setting. Open the "settings" app on your home screen, tap on the "cellular" tab, and scroll down to the very bottom of the list. There you'll see a toggle for "Wi-Fi Assist." Flick it off.
Consider this tip a Fortune assist.