So I have copied some folders with mp3s onto the watch, and created some playlists (*.m3u) for them.
Today I messed up some of my .m3u files, so they contained a few files/paths that were not on the watch (yet). The watch would reboot when I tried to upload these files - it should handle this error more gracefully, ideally giving some feedback about the problem or just swallowing it and ignoring any missing files.