* What led up to the situation?
Upgraded packages on 2015-05-04. Upgrades included kernel update from 3.16.0-4-amd64 to 4.0.0-1-amd64.
* What exactly did you do (or not do) that was effective (or
Attempted to use USB keyboard in, in Xorg and in virtual TTY.
* What was the outcome of this action?
There appeared to be no input from the keyboard. After disconnecting and reconnecting the keyboard it functioned; but after a period of seconds it consistently stopped working until it was disconnected and reconnected again.
* What outcome did you expect instead?
I expected keyboard input to work consistently.
* Is there a workaround?
I am able to work around the issue by overriding USB power management for the keyboard device:
cat on | sudo tee /sys/bus/usb/devices/3-1.2.2/power/level