As custom fields are created within an existing list of custom fields, the list can shift the custom field numbers of those above where the new field is created. If the custom field is referenced in custom email greetings, the display can be broken if the number is shifted. If a custom field can be "locked" so that the number doesn't change, this could be prevented. Of course, the field numbering drop down list would have to exclude the locked fields as new fields are created. This is not an issue for field testing with an Action Rule since the field is referenced by name.
