Apple's newest firmware for iOS devices has been highly regarded as the best yet thanks to many new features, tweaks and redesigns. However, there is one feature in particular that could be a huge issue for users with a limited data plan at their disposal.
The function in question is called ‘Wifi Assist' and when it enabled it will automatically harness your mobile carrier's internet connection when connected to a Wi-Fi signal that is deemed weak. This means you could be connected to a Wi-Fi network yet be wasting your valuable data plan without even realizing it.
I suggest anyone running iOS 9 without an unlimited data plan, navigate to Settings –> Cellular –> Wi-Fi Assist (very bottom of the page) and disable this setting. Turning this off will ensure your data plan is not being used without you knowing and keep you from any unexpected overage charges that may occur from the utilization of this feature.