Here's something you can try if this happens again guys... The screens on the S4 are VERY sensitive, and if something mildly conductive gets on the screen, it may register it as a 'touch' spot. Wanna try it? Get a droplet of water onto your screen and then try to unlock it. You can't (especially if the droplet is near the edges of the screen). But turn the screen off, wipe it clean, and try again and it should work if it was that. If it persists, then yeah, don't wait around and better exchange it for a new unit.