What happened to MySQL TIMESTAMP columns?

Imagine you are using MySQL for some application (not so hard to do) and you're running successfully on version 5.x.  For some reason you are forced to, or decide to upgrade to 5.6.  As always, version upgrades of your software's dependencies should be done with your eyes wide open and your tests ready to run.  As the title of this blog entry suggests, MySQL TIMESTAMP column behavior has changed is some dramatic ways.