Apple is investigating a bug in the iPhone 6S and 6S Plus that may cause the battery percentage level to be displayed incorrectly. More specifically, the percentage level on affected phones "won't update," which could lead users to assume their batteries are more charged than they actually are.

Apple's support document says the bug occurs when users manually set a new time zone or when their time zone changes automatically as they travel. The recommended fix for now is to restart your phone and to check that your iPhone's clock sets itself automatically (go to Settings > General > Date & Time to check).

This sounds like the kind of problem that can be fixed in a software update—Apple has a minor iOS 9.2.1 update and a major iOS 9.3 update in the works right now, either of which could bring a fix. Users who notice this problem and haven't changed their phones' time zone are encouraged to call Apple Support.