It would be a good idea in any training that teaches scaffolding drop-downs to remind people that in order to display an empty value, you must enter a non-empty string in the Empty Text property of the drop-down. Failure to do this will cause the first value of the drop-down list to display and that value will be saved as if you had explicitly chosen it, even if the attribute which is linked to the dropdown is not mandatory.

It would also be a good idea to automatically display an empty string and make the value a null identifier if the variable is a non-mandatory attribute.