How to recover Galaxy S4 from unrequested ADB recovery mode state?
I called up Verizon support and they had me simultaneously hold the three keys:


Volume Up
Power
Home


Please understand that this is the key combination for a Hard Reset on the Samsung Galaxy S4 and it will wipe your data from your device.

But after 30 seconds, still no response, so they had me remove the battery for ten seconds and reboot.  After reboot, the phone seemed back to normal.