A class-action suit has been filed against Apple in U.S. District Court over Wi-Fi Assist being turned on by default in iOS 9. Wi-Fi Assist is designed to switch to cellular data when a user is trying to perform an action over the internet on a poor Wi-Fi signal. This has the natural side effect of using cellular data. Since iOS 9 turned it on for many users, they weren't necessarily expecting that extra use, causing some of them to exceed their data caps. A former Apple employee who was in a leadership position for Mac OS X Wi-Fi software has commented on the issue, saying that the Wi-Fi Assist mess was unavoidable given how Apple's management treats that part of the business.

Quoting :"[O]ne particular directorial edict which I pushed back against at the end of my tenure sticks out as not just particularly telling, but deeply misguided: 'Make it self-healing.' Self healing in this context meaning that the networking system, Wi-Fi in particular, should try to correct problems that caused the network to fail, which, if you have spent any time trying to diagnose networking issues is a clear misunderstanding of the issues involved. ... Asking the devices which connect to this vast complex network of networks to detect, and then transparently fix problems in the infrastructure without the permission of the administrators is, well, it's absolutely the pinnacle of buzzword driven product management. Real pointy-haired boss territory."
http://apple.slashdot.org/story/15/1...s-wi-fi-assist