Several years ago a friend made a Bar Mitzvah and one friend didn’t get an invitation. The one that didn’t get an invitation didn’t know what to do. She didn’t want to ask , because then she would definitely be invited. She decided to go and just say Mazel tov and not stay. When she arrived, the hostess greeted her very warmly (no surprise at her coming) and showed her where the other friends are seated. She stayed for the meal.
About a week later the hostess called her up and said, “You can’t imagine how I appreciate your coming to my Bar Mitzvah. Your invitation came back today.”