Fannie Mae has announced that twenty one of their proprietary messages that were warnings will be upgraded to "hard stops" on January 26th, 2015. All of these new hard stops are overrideable, and you'll need to provide a reason code for the override if you resubmit without an updated appraisal.
In Mercury Network's UCDP interface, these new hard stops will appear alongside all other overrideable hard stops.
Fannie Mae's announcement includes all the new hard stop messages, as well as the override reason codes. Click here to get it.