I just encountered a lovely bug in ios 6 music playback.
I almost always unlock my phone to start/stop music playback on my iphone 5 (ios 6.1.4, still not upgraded to 7).
Last evening, I was in a hurry to pause it so I double tapped the home button to get the locked screen player controls. Later, I unlocked the screen, resumed playback and a few songs later I stopped it normally again, and USB cable sync'ed with itunes. This morning, I started playing one of my smart playlists.
I got 31 songs in and these were the results (I have linux on one computer and using libimobiledevice, I have access to what the phone is thinking and I can query the database right on the phone...)
The phone only registered having played 29 songs, the 30th and 31st songs did actually play, but simply stopped near the end of the 31st song (maybe it actually finished song 31). It refused to play by using the start/stop control until I rebooted the phone. When I did reboot the phone, it restarted on the 30th song, not the 31st song which it should have done... The current position in the playlist matched the database plays.
In my case, I would conclude that there is some code on the phone that says where it is in the playlist for playback (and possibly actually do the playing part), and there is somewhat unrelated code somewhere else that updates playlists and database. The code that updates, might have terminated/crashed/borked unexpectedly, preventing it from getting further play updates (after the 29th song in my case), while the other code that figures out where it is and plays keeps going. Maybe at some point the "player" is expecting the other process (maybe the GUI control, maybe another process) to receive its play updates, the other end never does, and the player plugs up eventually (2 songs in this case), causing a number of these problems mentioned in this thread.
If that is the case, it might also help explain why some phones (4, 4s) manifest so easily.... The specs on the 5 might delay, postpone or somehow alleviate the problem, but that it's still there.
Part of the problem encountered by people might be memory related, but I think it also has something to do with long delayed actions (you do something today that seems unrelated and 2 or 3 days later it manifests itself). If my numbers are an indicator, 30 songs would be a very rare album track count or maybe a very long playlist for some people.... maybe I poke the phone the wrong way this morning in the elevator???)
It seems my unexpected music playback issue I've encountered today (based on what I normally do and didn't do lately) happened to be double tapping the home button on a locked screen to control the player (on my phone that has a screen lock, I've had this kind of problem from time to time, I don't have a screen-lock on my ipod touch and even though was upgraded to 6.1.3 and I did not see that kind of playback reset problem on the touch)
I hope this helps.