This just had to happen to my boss
He has no delegates, no catched mote or offline working enabled anywhere. However now and again he doesnt get an email. It may be between a cluster of emails so its not time specific. He will notice an unread item when he gets to the office and when searching through his blackberry it wont be there. These are random emails and no real pattern. I have reactivated him twice by removing him from the BES altogether. However the problem props up now and again albeit rarely.