I'd like the ItemNotInList event to fire as the user works with my form, but if they decide to close the form with the X button, I'd like to bypass validation. Is there a way to do this?
No, if LimitToList is true, the control will not allow itself to lose focus when the entered value is not on the list. There is no way the control can know why it is losing focus, whether it be to another control or the close button of the form.