First noticed this in "The Invasion From Within", which has 3 custom audio and image combo-bursts. Using a skin with multiple combo-burst audio files (greater than the skin, in this case 4 audio samples) but a single combo-burst (or "<= map.comboburstImages") causes the surplus skin comboburst audio to fill in overtop the beatmap's customizatons. In other words:
map0.png/map0.wav, map1.png/map1.wav, map2.png/map2.wav, map0.png/skin3.wav, map1.png/map0.wav etc etc
so tl;dr: when custom map comboburst images/audio are present, skins should be ignored.
map0.png/map0.wav, map1.png/map1.wav, map2.png/map2.wav, map0.png/skin3.wav, map1.png/map0.wav etc etc
so tl;dr: when custom map comboburst images/audio are present, skins should be ignored.