This issue is the result of a record being present in either the GDB_ObjectClasses or GDB_FeatureClasses table for a table that does not exist in the underlying database. When this situation is encountered, the geodatabase upgrade logs a warning and continues with the upgrade. The warning appears as the following:
"Table (Fittings) was found in GDB_ObjectClasses, but does not exist in the database. Skipping."
There may be other error messages included with the warning.
This warning appears during the Pre-Requisite check portion of the upgrade process. If the Pre-Requisite check is run separate as part of preparing for an upgrade, an opportunity is provided to determine if the warning is valid and should be investigated. In most cases, the warning simply points to relic data in either the GDB_ObjectClasses or GDB_FeatureClasses table and can be safely ignored.