I was setting up mirroring at a client site today and started as I usually do by backing up the primary database and restoring it at the mirror. It would not start mirroring and complained that I didn't have the latest transaction log backup: the mirror database …., has insufficient transaction log data to preserve the log backup chain of the principal database.
Somewhere along the way, the need to have a transaction log backup has crept in. So, to start mirroring, I just now backup the primary database to a file, backup the transaction log to the same file and then restore both on the mirror server. Then it starts fine.
This seems pointless behaviour and it didn't used to work that way. I have an RTM SQL Server 2005 in a virtual machine and it works fine without doing this. Anyway, thought that posting it might help someone.