No matter the gadget, you'll always get a few duds off the production line that manage to get past QA and into the hands of an unwitting customer. Unfortunately for Sony, it looks like that number is much higher than a few, with what looks like a software bug causing the smartphone to temporarily die.
Going off reports from Sony's own Xperia forums, the phone will, sometimes fail to turn back on, despite having a full charge and being otherwise fine. Over at xda-developers, users believe the issue is caused by trying to turn the phone on too soon after having turned it off, an activity that's quite easy to do unintentionally.
The good news is if you do happen to trigger the bug, you haven't bricked your phone. It appears a cold reset (pressing Power and the Volume Up button together) rectifies the issue, but it's still an annoyance users shouldn't have to put up with.
Confirming that there is indeed an issue is this post from "Johan", a representative for Sony's Xperia Support team:
I'm posting just to say that the problem has been identified and a fix for this will be implemented in next software update for the phone. Unfortunately I don't have any exact release date for when the new software will be released but I will get back to you as soon as I know.
Thank you guys for contributing with your details and info, this always helps to identify any issues.
At least Sony is on top of the problem. A release date would be nice, though!