The "Delete Topic" button that shows in the button bar on Topic pages in Forums is confusing to site maintainers. Some users don't fully understand the distinction between a "Topic" and a "Conversation/Thread", so they have assumed that it is there to delete Conversations/Threads. They use the checkboxes next to each Conversation (used by the "Move thread" feature) and click that button and the subsequent confirmation screen assuming they're deleting only those conversations. Of course all the conversations are deleted instead, much to their dismay.
Per discussion on the sakai-user list, the (small) consensus was that button should be removed from that location. There are other existing ways to remove topics that are less confusing to users.