If your iPhone or iPad is running iOS 9, you may want to disable a new feature called Wi-Fi Assist. If you're somewhere where your Wi-Fi signal is deemed to be too slow or unreliable, it will switch to use your data connection.
The feature is meant to give you a more seamless experience when on spotty Wi-Fi networks, but this also means eating through your 500mb data cap without knowing it. You might think you’re safely Youtubing while on Wi-Fi, but instead it's using the 3G / LTE.
For more information, and instructions on how to disable the feature, see: