UPDATE: It looks like my suspicion about the cause of the problem is correct. Poor iTunes just got confused…

I keep my iTunes library on an Airport Disk, which works great but is most definitely the source of my problems here. Anyone who keeps his or her iTunes library on an external disk knows that if you don’t have that disk available, iTunes can’t sync it with a connected iPod. No brainer, right?

Well, with a regular iPod, this just means that nothing gets pushed out to iPod that isn’t available on the local disk. With iPhone, it ends up being a little more sinister.

Here’s the sequence of events, and it is entirely replicable.

1. I synched my iPhone in the beginning with a couple of playlists on iTunes.
2. I made changes to those playlists in iTunes and created a couple more for iPhone.
3. I synched iPhone with iTunes after making these changes. Changes indeed push out to iPhone.
4. Connect iPhone to Macbook while away from home network.
5. iPhone syncs playlists from step one back into iTunes! This also obliterates the playlists I created in step two, and the music available on iPhone reverts to the stuff that was available in step one.

This is a critical bug that has bitten me more than once now. If you are experiencing this issue on your setup, please leave a comment here or trackback from your own site.