Sounds difficult, but it isn't, and if you see such a message you will spot both the Subject change the the (was:..) block.
Remove "(was:...)" from Subjects
If you reply to a message containing "(was:...)" in the Subject,
- make sure you delete the whole (was:...) block contained in parentheses.
If the original Subject was "New cloud form discovered!" and the message where the topic changed had the Subject "The finest English umbrella (was: New cloud form discovered!)", then you should the Subject of your reply to this message to read "Re: The finest English umbrella".

