So that you can see when changes are made (either by me, the post's author, or someone else), topics now have a "Topic Log" that's only shown in the sidebar when changes have been made. It's collapsed by default, and you'll see a title like "Topic Log (3)" that you can click on to see the log of changes. I've added and removed a tag in this post so you can see what it looks like.
For now, this only shows tag changes and lock/unlock, but I'll add title changes and moving between groups shortly. Edit: This does not include edits to the post, you can already see when a post was last edited (if it was), but I don't intend to add more detail or a log for that.