I've been getting a lot of this error lately. I'm using Zune Pass, and all of my music downloaded using it. I've deleted the song, and when I want to redownload, Zune Marketplace says you need to buy this song (cannot be downloaded as part of Zune Pass anymore).
Well, then what's the use of Zune Pass? I was just shocked to know this morning that all of my Oasis song falls under this "delete-and-purchase" scheme. Even though Zune Pass doesn't actually grants us ownership of the song, we still need to buy for certain songs. Sigh.
Now the tagline "instant music collection" doesn't sound quite true anymore, probably more like "instant crippled music collection".
Come on Microsoft, you can fix this!
Zuning from Singapore!