Orphaned records in the Confluence Database can cause quite a number of upgrade issues, such as this error highlighted here:

2014-03-04 10:01:22,545 ERROR [main] [hibernate.tool.hbm2ddl.SchemaUpdate] execute Unsuccessful: alter table BODYCONTENT add constraint FKA898D4778DD41734 foreign key (CONTENTID) references CONTENT
2014-03-04 10:01:22,546 ERROR [main] [hibernate.tool.hbm2ddl.SchemaUpdate] execute ORA-02298: cannot validate (CON1_OWNER.FKA898D4778DD41734) - parent keys not found

This example is caused by BODYCONTENT orphaned entries that do not have references to the CONTENT table. A mechanism to sanitize these inconsistencies (that probably exist before proper foreign key constraints were introduced in an older version of Confluence, or probably caused by this: https://confluence.atlassian.com/display/CONFKB/Confluence+upgrade+errors+out+with+ORA-02298+parent+keys+not+found ) should be implemented to prevent upgrade pains. This can be done automatically during an upgrade, as part of an upgrade task, for example.