So say a question has been answered in the usual fashion, but by answering the initial question the user has ran into another problem which has been made apparent by a comment thread. The problem is very likely to be separate from the initial question and not related in any way.
What should we advise the user do?
In my mind, the original question has been answered - and therefore any issue that is not related to it is outside the scope of that question. It would benefit from a new question being asked, so that it would have the raised visibility to other community members, the comment threads aren't polluted with information not relevant to the new question etc etc..