Everyone was waiting for the Android 6.0 upgrade for the Shield after the K1 variant got it before Christmas. So while users were keen to be early adopters, it turned out that they probably wished they hadn’t.
A forum post from an Nvidia custom care rep explains that following the Shield Tablet 4.0 update, users might be unable to turn on WiFi. Those with WiFi-only tablets will be rather cross at this. It looks like the update is setting the MAC address on some devices to 02:00:00:00:00:00:00, and there's no user-facing way to fix it.
Nvidia suggests a number of troubleshooting steps like restarting the tablet, turning it off for a few hours, slitting the throat of your first born at a crossroads at midnight. A factory reset also appears to work wonders.
It is not clear why the problem happened to some users and not others. Nvidia decided it is much better to pull the whole update and start over. After all you don’t want to hack of Shield users – they are a nice couple.