One of Alexander's jobs was to design a ticket booth for the San Francisco public transit system. They had around 390 requirements for a ticket booth.
But if people started talking to each other, congestion would form in the queue – and there was nothing the designers could do about it.
Things emerge that screw with your design.
Realities emerge from the system itself. Can you create a configuration that is stable with respect to these realities?
A good design is free of inner contradictions