You'd think the concept of a lockscreen would be simple. It, you know, locks the phone. Several OEMs have still ended up with bugs that allow users to get around the lockscreen completely. The newest such vulnerability has been discovered in Sony's flagship, the Xperia Z. Just a few simple steps, and anyone can gain full access to the device.
In the video, you can see one Scott Reed demonstrating the problem. By pulling up the emergency dialer from the lockscreen you can enter the USSD code *#*#7378423#*#*. That code opens up the device's services menu. From there, it's a simple matter of triggering the NFC Diag Test, which allows you to press the home button and actually get to the homescreen.