I use Spotifyon my mobile devices (iPhone 6, Nexus 5, OnePlus One) and I keep having problems with the Android client:
On both Androids (4.4 and 5.x) the client most of the time is not reacting to the lock screen widget and in some cases crashing to have no more controls but out of a sudden starting. This is really annoying, especially when at work! The only way to turn of the music for such cases is rebooting the phone...
The problem even get's worse if the phone is connected via bluetooth headset at the same time. When the spotify app is running, the phone keeps connecting and disconnecting which casues the music to immediately to stop.
Is this problem already on the agenda for the next release?
I do have a similar problem with the Windows client and have posted a separate message.