The latest iOS 9 update from Apple has a rather sneaky feature that has seen people on limited data plans receiving sky-high bills.
It's called 'Wi-Fi Assist' and works by automatically switching to 4G if it senses that the Wi-Fi being used is running too slowly. When the new iOS update is installed, this feature is activated by default.
The good thing is, it's easy to turn off. This is how to do it: