Season 12 set... or is it?

So, I was thrilled to be able to find a brand-new, still-wrapped UK edition of the Season 12 set.  Hooray!  I opened it today... to find it contained the discs for Season 10.  Hm.  

There would seem to be two possibilities... either someone swapped out the discs and re-shrank the package, or this was a factory error.  What you do all think?  Has this happened to anyone else?  

I know Season 10 was released significantly after Season 12, which makes me wonder how it could be a factory error, but wasn't Season 10 re-released slightly later with corrected discs?

It is one of the (not so) rare misprints. Amazon UK was supposed to destroy all the misprinted discs, but instead sold them. So you've got the correct stories, just the wrong cover art on each disc. I can't recall if the BBC finally caved and sent out corrected discs.

Otherwise be proud that some collector could offer you a pile of cash someday to obtain a copy of this rare mistake that should never have been released!

Oh, that is a relief! That's great to know. Thanks very much!