Might be a bug with the Crystal furniture in the Dragon Lair? There's about a dozen spots to "display prisoners," in a dungeon, but when I send them back to the collection, it only allows one at a time. When I open the dialog, then exit (even without applying it to someone else) it removes the crystal from the previous.
Side-note, I've seen it happen on non-furniture equipment rarely also. The dragon crystal furnitures are almost every time.
Extra side-note: It would be nice to use the dragon crystals in the facility displays. The crystals are nice. ;-)

(Spoiler: from Jurassic Park)
