Has there been a first time? From my understanding to date there has not been any proven evidence that has been "hidden" away by encryption by terrorists.
In this case they have most of the info, obtained from apple under court order via the iCloud storage which Apple has full access to and can provide to Law Enforcement
In Paris the Terrorists used normal non-encrypted SMS
This entire FUD about "going dark" seems to be completely in the minds of the FBI and Prosecutors likely brought about by watching too many Bad Movies and TV Shows