Oh !!
After some testing I was able to reproduce your problem when I use an "single, simple discussion forum".
It seems to be a "bug" that toogles from showing menus to showing the subscription options.
I've seen that sometimes, the forum is called (in the url). Then you see the subscription options. Other times, the discussion is called (in the url). Then you see the menus.
I think that, in this type of forum, all the elements should be showed (the subscription options and the menus).
Can you confirm if this only occurs in this type of forum ?
If yes, could you send a bug to
http://moodle.org/bugs with the description of the problem ?
Now I believe in you error !!
TIA and ciao, Eloy