Why can't I add tracks to my starred list sometimes?

[classyjoes]

Sometimes I'm unable to add a song to my starred playlist, even thouth I can add it to other lists. 

Does anyone know why?

[andyfromks]

Yes, exact same issue.

For example, I couldn't add "We've Had Enough" by Ice and the Iced to my Starred list, but could add it to any other playlist I've created.

[andyfromks]

Just realized--I think it might be because that song is already in my starred playlist...   😕