Amongst its big bag of tricks, iOS 9 has one seemingly innocuous feature: Wi-Fi Assist, enabled by default, which will switch to cellular data when your Wi-Fi sucks. This would be great, if I had an unlimited data plan. I don't, but now I do have a very expensive mobile phone bill.
I've been using Wi-Fi Assist on my iPhone for a few months; even despite knowing what to look for (a greyed-out Wi-Fi icon), I haven't really noticed it in action. But it has been showing up in my cell data usage: since downloading the iOS 9 beta that introduced Wi-Fi assist, I've used around a third more data a month (4GB vs my regular-as-clockwork 3GB).
It's impossible to say if that extra usage is directly related to Wi-Fi Assist, but I have my suspicions. On the iPhone 6s that I've only been using for three days, my data usage is at 950MB; half of that is from Netflix, which I make certain to never use when I'm on the go. In fact, the only time I've used it in the past couple days was at home, using what I thought was Wi-Fi.
I don't know if I've just had one particularly bad experience, or a run of bad luck, or little gremlins sitting inside my phone chewing through cell data for no particular reason. But I do know that the Wi-Fi Assist toggle on my phone is being switched very firmly off. If you've got a limited data plan, and would rather not watch cat GIFs than burn through it all, I suggest you do the same.
To disable the feature, go to Settings->Cellular->scroll down to Wi-Fi Assist, and turn it off. (Or search Settings for Wi-Fi Assist.)