On build 2003012108, I forwarded some spam to myself as an attachment to make sure doing that includes full headers. It does. When you view the message and you have attachments displayed inline, you get the old-style header for the attached message, with Subject, From, Date, and To. These headers display the correct information. Now set your header display to full. Not only is it showing you the full headers for the current message, it is also showing you full headers inside the attachment. But the additional message headers it shows you are actually of the parent message, not the attached message.

Testcase for easy reproduction. Go into Mail/News and set header display to full. Now view this attachment. Notice that the headers after "To" of the attached message match the containing message in the display. View the message source to verify that the actual headers are different.

*** This bug has been marked as a duplicate of 140532 ***