Premium, it happened when I was free too.
My Question or Issue
This has happened twice now, I will listen to whichever music because I'm doing something on the side and have it playing in the background - then I check one of my playlists the next day to find I have 13 hours worth of songs added to it, all the ones I listened to last. This gets more annoying when the playlist is ordered in a particular way, and some of the music you listened to overlaps with the ones in the playlist, then you have to reorganize the whole thing.
If it helps, it only seems to happen on the one public playlist I have, all of my private ones never get this issue.