Logging into a SharePoint site after anonymously accessing the site causes the site to error.
When anonymous site is used in SharePoint the site includes a SIGNIN link, this signin link by default points to _layouts/Authenticate.aspx?Source=%2f
In the scenario:
At this point the site is no longer functioning even a restart will not allow access to the protected site "Server Error".
This turns out to be an FBA issue and not a SiteMinder specific issue.
CA Support work-around is to modify the link to point to a SiteMinder protected resource.