This problem occurred for me after I replaced the front facing camera on my 5S on 8/10/2014. The light on the edges of the screen would flicker on the lock screen and when the virtual keyboard was up. Turning my phone off and leaving it off for a few minutes didn't work. I went back in and reopened my case and removed the battery connector and reattached it. This seemed to do the trick. I think it may have something to do with not properly powering down the display if you remove the front portion of your phone (since you yank out the connectors for the face of the phone). I would recommend disconnecting the battery connector before pulling the front face fully off your phone. This is just also good tech advice in general.