We all know that the word "thread", when used here, implies "topic". However, there are questions about seeing topics in a threaded way, and that implies that the person asking the question doesn't entirely grasp the difference in concept and in structure.
As mentioned before, "threads" can branch and remain the same thread. Topics on the other hand strictly don't branch. It is possible that a post contains 2 or more partially different subjects, in which case quoting can reflect short deviations. However when these deviations become serious, splitting the topic into 2 parts, 1 being the original topic and the second a completely new topic (with a different topic_id), is advised. In threaded forums, splitting is seldom done (at least in my experience).
Transforming a threaded forum into a flat forum isn't as simple as it seems because splitting off branches usually destroys the flow of the messages,. Similarly, transforming a flat forum into a threaded forum requires strict and consistent quoting (which post is quoted?), which cannot be guaranteed in phpBB (as it exists now) (neither in other forum software, that I know of)
In my opinion the use of the word "thread" for topic should be discouraged in flat forums, if only to emphasise the difference in structure.
I do agree however (100%) with 3Di that a "reprimand" (I prefer the term "indication") should be a limited to single remark so as not to cause threading.