And sometimes, I just miss to select self-moderated and dont know how to change it after the topic is created.
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This has happened to me and I ended up leaving the topic to remain unmoderated. There is always a tendency that one could forget if your consciousness is not there.
Assume that I create a topic which isn't self moderated and you make a post. (You may not want to post in my thread if it's self-moderated). Then I make the topic self-moderated and delete your post.
This should not be a big problem.
The program be written that posts which were made when the topic wasn't self moderated shouldn't be possible to be deleted. The ones which were made after the post became moderated can be deleted.