OS X (desktop, installed) version only (that I've noticed):
I often am unable to add the currently playing song to a playlist; only after restarting Spotify is this possible (again). At some point, I am unable to add _any_ song to a playlist (via dragging, using the 'add to playlist' menu option, etc.).
Hello @puppyscent,
I'm marking this as "not an issue" for now as it's an isolated case.
Please try a clean reinstallation. This can resolve many common issues.
If the issue persists, don't hesitate to post in the Help - Desktop (Mac) forum.
Regards,
Dreadlord