Hi Lego Wargammer,
Sorry to hear that you had a disappointing experience with the Cafe Corner. I know how frustrating it can be to work so hard on a model only to find that you can't finish it due to missing pieces. It's very annoying.
I had actually heard of this issue with the 4x10 vs 4x12 dark gray plate through our consumer call center. However, we were able to determine that the plates actually were not missing but instead mistakenly used at another point in the model where the building instructions were not clear enough.
If you get a chance, it might be worth checking page 62 in book 2 of the CC building instructions. Several people have used the 4x10 plate where a 4x12 plate should actually be used. Another place to check would be the steps which show the construction of the floors, but this seems less likely to be the problem area.
It's extremely unusual to have large extra pieces with one of our models. So when this happens to me, I always go back through the steps to see where I might have overlooked something.
Of course, it could still be an issue where your set was not packaged properly, but I thought it was at least worth a try to check a known tricky spot in the building instructions.
Cheers,
jamie