A link on the dev tracker resulted in this message:
Invalid Forum Top specified. If you followed a valid link, please notify the administrator
The word administrator is a link, clicking it gets:
GlobalVariable, you do not have permission to access this page. This could be due to one of several reasons:

1. Your user account may not have sufficient privileges to access this page. Are you trying to edit someone else's post, access administrative features or some other privileged system?
2. If you are trying to post, the administrator may have disabled your account, or it may be awaiting activation.
No point in having it be a link if its not usable.

The original dev tracker item that triggered the invalid topic message seems to have been removed now that I refreshed the forum index, and was probably the cause.