I returned home one afternoon after work to discover my Pi was offline. After a bit of troubleshooting, I isolated the problem to a failed power adapter. The Canakit 5v 2.5a adapter worked flawlessly for nearly two years.
I promptly picked up a replacement adapter from the local Fry’s, this one a Rhino brand 5v 2.5a. I quickly noticed the lighting bolt symbol, and upon closer inspection saw the under volt message posted during the startup and shutdown process.
Believing the adapter to be faulty, I exchanged it for a replacement (same Rhino item). This one also displayed an under voltage problem. So I ordered another Canakit adapter thinking this would solve the issue, but, again, the under voltage indicator was displayed, along with the boot/shutdown message.
Since this only occurred following the June 2018 update, I am suspicious that something changed within the system that is causing more power draw. I have ordered an official Pi adapter (5.1v 2.5a), so I am hoping this fixes the issue.
As a side note, Canakit now includes an advisory sticker on the adapter box stating any under voltage issues are due to an incompatible operating system …