If a forum has been set up with separate groups, the user may get a 'Next topic' or 'Previous topic' link to a discussion posted for a different group.

To reproduce:

Create 2 groups (group A and group B) and add some students to each group.
Add a general forum, with group mode = separate groups.
Add a discussion topic for group A.
Add a discussion topic for group B.

Log in as a student in group B.
Navigate to the forum - the student should see only the discussion topic for group B.
Click on the discussion topic.
Note that there is a previous topic link to the discussion posted for group A. Clicking on the link results in an error page that says 'You do not have permissions to view this post'.

The next or previous topic in a forum should point to the next or previous topic that the current user has permission to view.