ORA-12514: The Database Is Up. The Listener Just Doesn't Know It.

What ORA-12514 'TNS:listener does not currently know of service requested in connect descriptor' really means, the four common causes on a healthy database (a wrong service name, a listener restart, an app service that didn't come back with its PDB, a listener the database doesn't register with), how to tell it apart from ORA-12154, ORA-12541 and ORA-12505, and a lab where restarting the listener causes a minute of ORA-12514 that ALTER SYSTEM REGISTER closes at once.