As you know, there are many Categories and Sub-categories on forum.
Users are allowed to post under main categories as well as sub-categories. I am not sure if it's possible, but I think users should not be allowed to add discussions or questions to main categories.
Let me give an example to be more clear. You can post your query about Nokia 8 directly to Phones section instead of Nokia 8 sub-category.
For a small community it maybe ok, but for a large community like this to follow the posts and categories is becoming very difficult. Furthermore, it is allowing users to post into the wrong threads and making things difficult for moderation.