ROLL_INVALID_MISMATCH (external link)

Great article on one of reasons to not share cursor https://magnusjohanssontuning.wordpress.com/2016/04/22/cursors-not-shared-due-to-roll_invalid_mismatch/

Password hash explained (external link)

An excellent article on password hashing in Oracle https://www.trustwave.com/Resources/SpiderLabs-Blog/Changes-in-Oracle-Database-12c-password-hashes/?page=1&year=0&month=0

Insightful article on connections db terminated by firewalls (external link)

Very insightful article on why/when firewalls throw connections https://li555li.blogspot.se/2014/09/.
In short - to prevent this one have to set SQLNET.EXPIRE_TIME to {timeout on firewall}/2-1 in minutes.