How many times have you tried to open a door and found out it is locked? You quickly apologize for not knowing the room was occupied, but was it really your fault? How could you have known the door was locked?
Doorknob Condition by Arnaud Lapierre is designed with privacy that is intuitive. To lock the door inside, the user simply pulls on the door knob.
This causes the door knob on the outside to retract.
Now anyone outside clearly knows the door is locked, since there is no physical affordance for opening the door.
To ensure good design, the door knob should only be retractable from the inside. Further, in case of emergencies, there should be an easy unlock mechanism from the outside.