Sometimes new messages come in and Mail (on OS X Lion and Mountain Lion) shows them as empty, with "This message has no content" in the preview column. The message is empty in the full message view window.
I'm connected to Exchange 2010 on the backend.
When I open Outlook for Mac I can see the mail, and it's definitely not empty.
When I quit and restart Mail the message appears in its entirety, but the middle Mail column (with the subject/from/snippet in it) still says "This message has no content".
So a Mail restart sort of fixes the problem, but it's quite annoying.
I didn't have this problem pre-Lion with the same Exchange backend.