Apple's new iOS 9 introduced a new feature. It automatically uses your mobile data if wifi network was slow. Sounds like a handy feature until you realized it costs 10c a MB or more to use mobile data.It’s called Wi-Fi Assist . So how do you turn this thing off?
It's under Settings -> Mobile -> Scroll to the end of page (Wi-Fi Assist) -> Turn it off.
No more bill-shock.