Some Sony Xperia Z users are reporting that their device is randomly shutting down, never to turn back on again. XperiaBlog points to several different user groups that are finding their devices shut down without their knowledge, and can't do anything to have them come back to life. Although some report success doing a hard reset (holding power + vol up), this doesn't seem to be universal fix. There also doesn't seem to be a known cause either, which makes the bug even more troublesome to diagnose.
Sony's response to the issue is that it has come up with a fix for the random shutdowns, which will be implemented with the device's next software update. Our own Xperia Z seems to be going along strong and hasn't had such an issue, but then again we may just be lucky.
Source: XperiaBlog; More: Sony Xperia Z Forums
