You're an administrator, and you've installed a few servers running Microsoft Exchange Server 2010. You know that mailboxes are stored in mailbox databases, and, based on previous versions of Exchange, you know that you need to specify a mailbox database when you create a mailbox. One day, you go to create a mailbox and forget to specify a mailbox database. And you don't get an error. And then you ask “Where’d it go?". You eventually discover that the mailbox was created on some random mailbox database that you didn't specify yourself. You wonder what in the world just happened. Exchange administrator, meet automatic mailbox distribution.
Read that interesting article “Where Did That New Exchange 2010 Mailbox Go?” at http://technet.microsoft.com/en-us/library/ff872148(EXCHG.141).aspx