WiFi Assist is a new setting is meant to help your connectivity, but it could cost you money.

In Apple iOS 9, there’s a new feature called Wi-Fi Assist. When you’re using a spotty Wi-Fi connection, it switches over to cellular data. (Typically, your phone switches over to a cellular connection when Wi-Fi becomes unavailable. With Wi-Fi Assist, it kicks in when Wi-Fi meets what it deems as a “poor” connection.) If you have an unlimited data plan, it’s no big deal.

If you have a data cap or shared plan (most people do) you may unknowingly be using cellular data, and blow past your limit. That can cost money.