You may have run across a situation where keywords you are quite sure you deleted from the Keyword list, and therefore deleted from all photos, are suddenly reappearing in your kewords list.

It turns out that as part of repairing or rebuilding your database (see FAQ: “How do I repair or rebuild my Aperture library or permissions?”) Aperture will extract old keywords from the Apeture preferences, or .plist file.

So to do a clean rebuild and not see old keywords return, you should delete the preferences first (see FAQ: “How do I trash my preferences?”).