Steps to reproduce:
Play a map with a beatmap specific skin and difficulty specific skin like http://osu.ppy.sh/s/14224
Disable skin with Funspoiler, press escape and hit retry. Osu will now ignore the beatmap specific skin and keep the difficulty specific skin.
Exit out of the map and go back to the song select screen, then load the map again. Osu will now ignore both custom skins.
Play a map with a beatmap specific skin and difficulty specific skin like http://osu.ppy.sh/s/14224
Disable skin with Funspoiler, press escape and hit retry. Osu will now ignore the beatmap specific skin and keep the difficulty specific skin.
Exit out of the map and go back to the song select screen, then load the map again. Osu will now ignore both custom skins.