I noticed a very annoying problem implicating the using of Spotify offline. Often playlists saved on my device are not available offline so for example when I open spotify on the plane or when I don't have connection I cannot listen to the whole playlist. Why does the playlist always have to be updated through the using of the internet? and even if some songs are not available, it's crazy there's no possibility to listen to the other songs correctly saved.
Another problem happens sometimes when I open spotify without internet connection and it asks me to sign in! And of course I can't do it without connection. I think they are very irritating problems, it's useless to have spotify premium if I cannot listen to the playlist saved and if the app asks me to sign in when I'm offline.