I run Outlook 2007 and have never had this problem.
A good friend of mine runs Outlook 2010 (also on 64 bit win 7) and has a weird problem. He claims (though I have not seen it myself) that sometimes, after he receives a number of emails (prior to reading them) that a single email will "disappear" after re-opening Outlook at a later time and then later reappear all by itself the next day when he runs Outlook again.
Although this doesn't sound terribly credible he has told me that this has happened three or four times. Does this ring any "Bells" out there - Any ideas.
He wants my help in fixing this.
For now I told him to carefully record the Address and any any other noticeable details of the emails that supposedly disappear/reappear to see if their is anything in common between them.