From what I can tell, if a Google Groups user wants to create a new topic, but instead of logging into the web site or opening a new email and addressing it to the group address, they just hit reply to an old thread and change the subject line, Google Groups will get confused.
For example, you can check out this thread (this isn't the thread that really made me ask the question, since the group I was using when I thought to ask is not publicly accessible, but this illustrates the point).
Is it possible to fork off the thread into its own proper discussion, so the web interface doesn't look so confusing?