Spotify playing songs out of my playlists

[Kbmelo]

When I play some of my playlists, between some songs the Spotify (free version) plays songs that I didn't add in my playlist, songs that aren't even from the same artist/band or genre.
I'd like to know if this is a bug, something I can fix or maybe something I don't know.

[Hubo]

Hey! This isn't a bug. Spotify Free plays music related to the music from your playlist between tracks.To listen only the music you like you have to upgrade to Spotify Premium. I'm sorry. You can read more about Shuffle Mode here, see the question 'I wanted to play a specific artist album. Why am I hearing different tracks?' Good luck!

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[Kbmelo]

Thank you, mate. But that's not something that I always see happening, is there a rule, for exemple: if your playlist has more than X songs, it won't happen. I don't really see it going on with my big playlists.