IF
you have enabled Tell me when all messages have been read in Outlook,
                    
AND
you send a meeting request to a user that has defined a delegate to receive their meeting requests,
                    
AND
this user does not receive the meeting requests,
                    
AND
the delegate selects the option to delete accepted meeting requests,
OR manually deletes them after processing,
                    
THEN
you receive the subject message.

NOTE: I know of no way to prevent this (unless you clear the Tell me when messages have been read check box).