Help!

Yes, I find that for categories, you sometimes need to purge the cache or save the page again but make no changes or something else. I don't really know at all how the software caches pages.

The issue with longlegs was actually not that; it was that before Martin deleted it, it had {{dervcat}} in it and looked like it should have been named Category:English words derived from: longlegs.

Even cache-purging didn't seem to work. But, when I came back some hours later all was cool.

I'm glad MG cleaned up my little mess. I have to revisit most of the {{derv}} entries and associated categories because the ideas are still evolving a bit. So I will have plenty of chances to find redlinks etc.