Yeah -- unique and set items are very unstable. I've seen all these behaviors:
- item disappears
- item changes to a plain item with white text
- item is normal in inventory, but doesn't appear in trade screen
Fortunately I've found plenty of great dark orange equipment, so don't feel the loss too acutely <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" /> Still, it would be nice to know what set items DO, and to be able to collect a set!