On slim-modeled custom skins the right leg is being copied and flipped onto the left leg (but not the second layer). Here are some pictures as examples. The ones with the grey background are the ones that are the way they should be, and the ones with the green, Minecraft background are the glitched ones. The second layer to the skin is fine but the first layer is the glitched one. This only happens on slim models for all I know.