When viewing a forum or topic as guest the buttons to reply or post a new topic are present. They will redirect to login page and then to posting pane once logged in. This is desirable IMO. It's convenient for registered user and unregistered user gets a hint to register to post.

The bug is where forums are viewable by guests but only postable by special groups, the Announcement forum as one example here on phpbb.com. In that case when a registered user logs in instead of posting pane they get a message "You cannot post in this forum".

Minimally  I would suggest registered user permissions are checked before displaying the buttons to guests.