WASHINGTON — A feature of iOS9, Apple’s new iPhone operating system, has the potential to eat up data before the user realizes why or how it is happening.

“It’s a real problem,” says WTOP Tech Guy Gregg Stebben.

The feature is called Wi-Fi Assist, and it’s supposed to help Wi-Fi users keep a stable Internet connection by switching their devices automatically from Wi-Fi to the cellular network whenever the Wi-Fi signal becomes weak.

This may be convenient, but it can easily lead to unexpected expenses.

If the user does not realize the switch from Wi-Fi to network has happened, he will be using data without even knowing it — until he sees his bill.

“A lot of people are already aghast because you realize, ‘the reason I’m using Wi-Fi is so I don’t use my data plan, and all of a sudden you are telling me without even knowing it that Apple is deciding that I should use my data plan,'” Stebben explains.

Stebben recommends turning off Wi-Fi Assist and keeping “very tight control of when and how you use your data plan.”

Users can find the feature by going to Settings, then to Cellular.