Before just saying this is a duplicate issue let me first say; this is not the black lines you get when using a slim arm skin on a standard model! When making a skin in the classic layout, 64x32, and using a slim arm model, consistent black lines and shifted pixels occur. This issue is not visible in the launcher when selecting the skin, or when the skin is rendered on, at least some, 3rd party websites, however in game this issue is present. Pretty much certain its an issue with the arm being mirrored improperly in-game as when it tries to mirror it starts from 2 pixels to the right of the slim arm; weird it doesn't do that in the launcher though.

(side note; I did realize while making this that there is one additional line of pixels on the green skin, that's why the black bars were of different thicknesses in the 2 skins, testing with some other skins confirms that, but even so it is not responsible for the issue itself.)