I've never gotten this. Either the subject still has relevance, people are practicing sound thread conservation, or someone wants to comment on an old topic, even if it has been resolved and yes, sometimes they did not notice it was an old thread. When I ressurect old threads, usually I add a comment about it at the beginning of my post, to appease the critics, but it depends. If it's a thread with as much relevance today as it had a couple of years ago, I might not. If it's someone else who ressurected the thread and did not note it, I might say "ressurected thread" in my reply if I make a repy to add to the discussion.