Sometimes (I haven't been able to manually reproduce it) the widget (with pause/next buttons and song info) in the notification bar and lockscreen will crash (the buttons won't respond) and I'm not able to swipe it to remove it. Then I will close the app but the widget will still keep there, crashed. The only way to remove it is to reboot the phone.
I'm using a Galaxy S6 with Android 6.0.1 and this has happened to me with several Spotify versions, including latest.
Any ideas of the cause? Thanks.