I was working in a project where we are using database mirroring as high availability solution. In the same instance we have multiple databases in which only critical databases are configured for database mirroring. As you know database mirroring scope is limited to database only which means dependent database are not failed over (which is now possible in SQL Server 2012 as Availability groups). In my case the dependent database is not even configured for mirroring. They have some procedures \ views which will pull the data from dependent database. Assume if the mirrored database is failed over to next node they won’t be able to pull the data from dependent database which means they are screwed and application will be down till we go back to principal node.