Mine did the same thing when I first got it. Within the first day I think.
I had it set to where you draw a pattern over the 9 dots to unlock it. The center dot was red and there was a line shooting in all different directions, but not making contact with the other dots.
Even when I tried to unlock it myself, it kept getting the pattern wrong. I shut the phone down, rebooted, and it hasn't done it since.