The Wi-Fi Assist feature in iOS 9 was meant to save consumers from shitty Wi-Fi by switching to mobile data. Instead, it's eating through data caps, and now apparently causing lawsuits.
A California couple is suing Apple for $US5 million in damages relating to Wi-Fi Assist. The suit alleges that after upgrading to iOS 9 on their iPhone 5s', the couple saw overage charges because they had unknowingly breached their data limits. Because of the number of people (in theory, most every iOS 9 user) affected, the suit reckons potential damages are in excess of $US5 million.
The lawsuit itself can probably be filed alongside McDonald's coffee in the Hall of Frivolous Fame, but it does highlight that Apple faces a real problem with Wi-Fi Assist and mobile data caps. After iOS 9 came out, a whole legion of articles were penned pointing out that the update was causing an increase in data usage.
Even a former Apple employee who worked on OS X Wi-Fi products has written an article, agreeing that it seems to be a problem, and highlighting Apple's corporate policy as the culprit.
As a reminder: if you want 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.)