I had some serious problems with my iTunes syncing Playlists over the weekend which took a while to sort out.
It related to the Play Doctor list obliteration
I reported over here. As I mentioned in that thread, I went through and fixed them all. While I was at it, I fixed some other small issues with my playlists used to sync MC over to iTunes so that I can sync our iOS devices.
The next day, I tried to sync my wife's phone to iTunes. I noticed immediately that one of those fancy Shuffles suddenly contained every track in my Library, rather than the 100 Play Doctor generated tracks it was supposed to contain. I went and looked at my Playlists, and I noticed all sorts of craziness. All of the lists that I fixed were there, but all the Playlist is all [Some List] references were blanked out (replaced with a broken Playlist expression thing). And, my main "All Worthwhile Music" list (which contains all the music I want to sync over to iTunes, and which I'd tweaked) was still there, but there was also a new Smartlist (called New Playlist) next to it which had no filters at all, and so contained all the video, images, and audio in my Library. Any Smartlist I had that referenced the All Worthwhile Music smartlist also had their Playlist is any X settings lost completely (which, in most cases, caused them to show all files in the Library). Nothing actually referenced this New Playlist, but it was sitting there in my Playlists\iTunes Lists\Automatic Lists\ folder, and all the references I did have to things in that folder were broken.
Same goes for the Handheld Sync options. In there, that New Playlist was checked, and my All Worthwhile Music list was unchecked.
I
never manually created that list, and I certainly never checked it in the Handheld Sync options.
I deleted it, fixed my lists (again), and resynced. Everything looked fine for a few minutes, but then while I was still sitting there tagging some other files, I saw this same New Playlist item reappear. I quickly went in and checked my other Smartlists, and they were again all broken. This same thing repeated a number of times.
After much experimentation, I discovered that this was because the HTPC upstairs was connected to the Server while I was making these changes (my wife was watching TV). If I closed the client upstairs, and then fixed everything, then it "stuck". But as soon as I opened up the client and left it running upstairs, any changes made to my Playlists downstairs turned all crazy. It looks like, technically, that:
1. New playlists do get saved.
2. Playlists cannot be deleted reliably, nor can they be moved from one folder to another.
3. Playlist to Playlist references (Playlist is any X or Playlist is not X, for example) get broken if they're changed in any way, even the ways that normally "automatically fix themselves" (like moving a playlist from one folder to another, or renaming an existing one).
4. If a Playlist has a reference to another Playlist that is broken by #3, then MC will create this "blank" New Playlist to fill the void (but then doesn't actually link it, as the links are still broken).
To be clear, this all applies when making the changes
on the server (not from the clients). The Library Sync merging is getting all out of whack.
I remember something similar with MC17 in the early days. I suspect this might have something to do with the Playing Now saving system, which has changed. Not sure, but it is all broken currently.
Right now, the ONLY way to do Playlist management of any kind on MC is to do it with all Clients disconnected and shut down.