Chrome Version: 57.0.2987.32 (Official Build) dev (64-bit)
OS: ChromeOS

What steps will reproduce the problem?
(Sorry; no idea what specifically triggered this condition)
(1) Leave device idle for a while, so it sleeps, and locks.
(2) Wake the device.

What is the expected result?

Expect to be prompted with a little card with my happy smiling[1] face on it, allowing me to enter my password to unlock the device.

What happens instead?

The screen is blank, aside from the few icons normally found at the bottom-right of the tray area (clock, wi-fi indicator, user thumbnail, etc).  Clicking the tray menu UI showed that the device was clearly still locked.

So far this is a one-off, but I've never seen this issue before, so tagging as Regression.