Weekend Learning: EBS 12.2 login issue when DB upgraded to 12c

Combining upgrades for both the Oracle E-Business Suite and Database, the MOS document "EBS 12.2 Login Issue after Database Upgrade to 12c - The system has encountered an error when servicing the request with initialize class oracle.apps.fnd.common.WebAppsContext error" with note ID 1987505.1 shows a very simple reason for WHAT caused the problem but not WHY it happened at all!  Check it out to see what went on and how to solve it.