The Wifi at my work requires you to login before being able to browse the web.  If you attempt to visit any url without first signing in it redirects you to a "webpage unavailable" message.  I would expect Spotify to fail to download songs, but it doesn't.  Instead any song which downloads while I'm on the Wifi but not signed in plays with no audio, and sense there isn't an easy way to redownload a single song I end up having to redownload the playlist of some thousand songs.

It wouldn't be a particularly hard fix, validate the download with some hash on completion, and don't attempt a redownload until the network state changes if it failed.