Do you have a link for that thread, Writin_Reg?
The only wallpaper I can find on those three lots that isn't available in the catalogs is the interior of the foundation:
http://i7.photobucket.com/albums/y288/Bluefunk/Sims%203/Sims3UnavailableFoundationTexture.jpgHowever, that foundation texture is used in almost every EA lot in Sunset too, so that wouldn't explain why the corruption would only affect those three lots. I'm guessing that for that foundation, the lots were probably made prior to the brick texture being coded as the default for the foundation walls.
There are some plain walls that have a strange issue though- in the art gallery for example, if I select some of the solid coloured walls with the CAST tool I can see that their wallpaper type is in the gallery ('flat wall') but if I look at the swatch, there is no material category (it goes to whichever category you had selected last). Instead of showing up as a highlighted pattern, or at the bottom as a 'custom' (recoloured) pattern, there is nothing selected. This screenshot shows the red wall, with the 'flat wall' wallpaper style highlighted, but no pattern selected (top right by my cursor arrow). I had had a 'miscellaneous' category item selected last, but if I selected 'geometric' or any of the others then it would remain on that category when I CAST selected the red wall.
http://i7.photobucket.com/albums/y288/Bluefunk/Sims%203/Sims3UnavailableSwatchTexture.jpgPerhaps these walls were coloured before the game was finished, too- and maybe it is those patterns which are becoming corrupted. Doesn't explain why some people are seeing this in every instance of Sunset that they create, and others (like me) cannot make it happen.