Though people have to go out of their way to trigger it, a time-related glitch discovered in iOS 8 and 9 will let users potentially "brick" many Apple devices, rendering them non-functional.
Users must go into the Date & Time menu under Settings, disable the "Set Automatically" option, and then manually roll the date back to Jan. 1, 1970, according to a YouTube video published on Thursday. This requires jumping in and out of the menu, since the date won't initially rewind that far.
Once the new date is set, rebooting a device may leave it stuck at the Apple logo, and even the full range of restore options may not work. In that circumstance there's likely no option but to get a Genius Bar repair or a replacement device (possibly free under warranty), though some people have reported that a device might spontaneously recover several hours later -- albeit with extremely slow performance. That may allow users to change date settings and recover.
A Reddit thread suggests that the bug may be related to timezones, since rolling the date back as far as it will go could be setting the clock to a value less than zero. One person said they only bricked their phone after changing timezones from Beijing to New York.
In addition to requiring iOS 8, the bug is also limited to Apple devices with 64-bit processors, from the iPhone 5s onwards. Older products like the iPhone 5 or iPad 2 should be immune.