This error appears in the logs when there's two items in the database:

2008-12-20 18:17:23,697 ERROR [http-8080-Processor4] [com.atlassian.hibernate.HibernateObjectDao] findSingleObject Uh oh - found more than one object when single object requested: [globaldescription: v.1 (29687809), globaldescription: v.1 (29687810)]
– url: /login.action | userName: anonymous

The workaround is to delete one of the duplicate rows from the Content table, with ContentID being the value in parentheses.