I asked Nokia and they said they are not sure what it could be but they hinted accidental damage and send it in. Sadly i'm on Dev Pre 8.1 and they won't accept it with this installed. Gonna have to wait to till it is officially released.
Liam, who told you that they wouldn't cover the phone under warranty with 8.1 on it?
A Nokia rep told me over the phone a couple of weeks ago that since mine had 8.1 on it, the warranty was voided. I needed to send my 1520 in for repair. She did let me know, however, that all I had to do to get the warranty back was to use Nokia Software Recovery Tool to flash WP8 back on...and no one would ever know. According to the rep, Laura, the fine print of the Preview agreement does, in fact, state that using it voids the warranty. I still haven't read it though. ��
They told me the same but i own a Mac and not a windows pc (don't ask) so I can't revert, just have to wait.
Tip to all, as wp 8.1 is slowly being released their is a way for the windows phone 8.1 developer preview in the about page within settings to be removed.
First, uncheck your preview for developers so you're opting out.
Then hard reset the phone, hopefully on the about page it should just show windows phone 8.1 (its worked for me)
I don't think it has ever said Preview for Developers in the about section of my phone but I did do a hard reset on my phones after installing 8.1