Custom.pll template are sometimes used wrong. In case of 12.2 make sure you use the splotchy 12.2 template to put in you customizations. In case if you are migrating from a older release make sure it is created from the latest versions. You can check the version using adident package. ORA-06508 are known to be a problem with older custom.pll template.
$APPLTMP directory in R12.2 if placed in a shared location in a multi node environment the there are know complications in multi node cutover. So best to leave it under INST top for each node.
Run EBS 12.2 Compliance Checker and resolve any violations.
Document 1531121.1, "Using the Online Patching Readiness Report in Oracle E-Business Suite Release 12.2"
The consequences of failing to fix a violation depend on the type of violation. Objects that do not comply with Online Patching development standards may behave incorrectly or become invalid during or after online patching.
Some customers have synonyms from custom schemas pointing directly to the base tables. Don't (more...)
Wish every one a Happy New Year 2015.