The code that deals with the media keys on the Mac is used by several other programs and because of the way the Mac works, has to keep a "database" of other programs that use the media keys.
Here's what it currently has:
@"com.spotify.client",
@"com.apple.iTunes",
@"com.apple.QuickTimePlayerX",
@"com.apple.quicktimeplayer",
@"com.apple.iWork.Keynote",
@"com.apple.iPhoto",
@"org.videolan.vlc",
@"com.apple.Aperture",
@"com.plexsquared.Plex",
@"com.soundcloud.desktop",
@"org.niltsh.MPlayerX",
@"com.ilabs.PandorasHelper",
@"com.mahasoftware.pandabar",
@"com.bitcartel.pandorajam",
@"org.clementine-player.clementine",
@"fm.last.Last.fm",
@"fm.last.Scrobbler",
@"com.beatport.BeatportPro",
@"com.Timenut.SongKey",
@"com.macromedia.fireworks", // the tap messes up their mouse input
@"at.justp.Theremin",
@"ru.ya.themblsha.YandexMusic",
@"com.jriver.MediaCenter18",
@"com.jriver.MediaCenter19",
@"com.jriver.MediaCenter20",
@"com.jriver.MediaCenter21",
@"com.jriver.MediaCenter22",
@"com.jriver.MediaCenter23",
@"com.jriver.MediaCenter24",
@"com.jriver.MediaCenter25",
@"com.jriver.MediaCenter26",
@"co.rackit.mate",
@"com.ttitt.b-music",
@"com.beardedspice.BeardedSpice",
@"com.plug.Plug",
@"com.plug.Plug2",
@"com.netease.163music",
@"org.quodlibet.quodlibet",
If you are using something not on this list that uses the media keys that might be an issue.
The reason each MC version is on there is it will allow for running more than one version. Not common but there is very little overhead.