You probably want to be setting NotifyForm(4) on Form Entry rather than Form Exit. The reason for this is say you have two records, one has a ticket_number and the other one does not. If you leave the record that does not have a ticket_number, and go to the record that has a valid ticket_number, the On Form Exit event runs. Since on the record that you are leaving ticket_number is blank, NotifyForm(0) gets called and allows the user to delete records. This means they can delete the record that has ticket_number filled in.

-Ray