Blender is always buggy, not just Blender, also Maya, 3DS Max, Zbrush, Windows, Mac OSX, that’s just how computers go. Blender is great when it comes to bugs compared to other software’s. Those black lines might be to a modifier, try turning them on and off and see if they go away. Perhaps is a flipped normals issue or just as simple of going in and out of edit mode. Try things around, maybe download a fresh Blender version, because I’m not getting them when I open your file here.
