(slot-missing (class-of instance) instance slot-name 'slot-makunbound)
(Any values returned by slot-missing in this case are ignored by slot-makunbound.)
The specific behavior depends on instance's metaclass. An error is never signaled if instance has metaclass standard-class. An error is always signaled if instance has metaclass built-in-class. The consequences are undefined if instance has any other metaclass--an error might or might not be signaled in this situation. Note in particular that the behavior for conditions and structures is not specified.