I have found a slightly annoying bug on Spotify for iOS that causes music to speed up (approx. 2x speed). It has only occured twice, but I would like to make Spotify's developers aware of the issue so they can solve it.
The first time, I had paused Spotify on my iPhone, then later hit play on a bluetooth device I had connected to my phone. There was no response the first time I hit play for several seconds, so I hit it 2-3 more times until the music started. The music playback was at around 2x speed for several songs (I wanted to see if it would fix itself) until I started a song over, which fixed it. This was on version 5.3.0 of the Spotify Music app on my iPhone 6 Plus.
The second time, I had been listening to Spotify Music on my iPhone, and changed the song on the Joy for Spotify app for Apple Watch. I then decided to go back to the song I had previously been listening to, so I opened the Spotify Music app on my phone and the song I had previously been listening to was still there, paused, with a dialogue above it that said something along the lines of "Your music has been paused because your account is being used by another device." I dismissed the dialogue and hit play, and the song played at around 2x speed. I started the song over, and again fixed the problem. This time I was using version 5.4.0 of the Spotify Music app, on the same phone.
This problem is easy to fix by starting the song over, but I shouldn't have to start my music over every other time I try to listen to it. I hope my reporting of this bug will assist Spotify's iOS app developers in finding and fixing it. Feel free to ask for any other details if you need to. Thanks!