Slimes spawn on surface

Strange as it sounds, slimes spawn on the surface of my survival world. I created the world in 1.4, but I also created the world in windows 7 pre-updating to windows 8 and the bug seems to have appeared after the upgrade. Screenie attached, with debug screen turned on (for further debug information). Mods installed are optifine and I launch with magic launcher, but as I started using those after the bug appeared I don't believe they are related.