This is kind of a nasty one. I've verified it on two separate CS2007 installations. One is pre SP1 and one is post. Please note that this bug completely takes down the site and requires database modifications to fix, so proceed at your own peril.
Steps to reproduce:
1. Create a new role (make note of the GUID assigned to the role id, so you can get the site back up and running afterwards).
2. Go to the user permissions screen for any forum and add any permission to the new role. I just chose "View"
3. Go back to "Manage Roles" and delete the role.
4. Exit the control panel. Unless my installations are messed up, you should now see Role Not Found, and the site is essentially inaccessible.
The cause appears to be that the role is deleted from the aspnet_Roles table, but it is not properly deleted from the cs_SectionPermissions and cs_ProductPermissions tables. You will need to go into these tables and manually delete the rows corresponding to the GUID you hopefully recorded in step one.
Matt