To paraphrase from the guideline published for The Jackrail http://www.jackrail.org, please do not deviate overly from the subject described by the Topic. By all means create a new Topic with a meaningful name to reflect any change in content.
Replying to a topic post with a completely unrelated subject is called thread hijacking, and it really is generally frowned upon. There’s a three hour lecture I could produce on why this is a Bad Thing, but you can read our posts about it on The Jackrail, and indeed, almost any mailing list.
We’d appreciate it if people were cognisant of this. Topics are a powerfully simple way of classifying discussions, and when they go off track, they lose a lot of their value, for archived searches for example.
I think the point is that a list and a forum are not really just a chat room where one thing leads to another ad infinitum in a long chain of replies. It should be somewhat more structured. [But I made a category for unstructured chit chat and hello’s: The Pub.]