Another thing that generates the transaction failed or fare no longer available message is not being logged into the Delta.com when trying to book a flight actually offered by Northwest. It used to be that you couldn't book a ticket for anyone else when you were logged in, now they've made an unannounced change that you have to be logged in. But nobody in help desk or in their Web design page realizes it.

Of course, major changes are coming shortly as they purport to finish the Delta-NW integration so who knows how it behaves next time.