Google's commitment to Android in the form of monthly updates for its own branded hardware is pretty great... until it's not. That's the case with the May security and stability update for the top-of-the-line Pixel C tablet, which has created some serious headaches for owners. Some (but by no means all) owners of the Pixel C are reporting more or less random reboots of the tablet, usually occurring every five to thirty minutes when the Pixel C is off its charger.
Our fearless leader Artem Russakovskii has experienced this problem. (Of course he has - if there's a technical failing with any product and Artem is within dead cat-swinging distance, he'll experience it.) It seems possible that the issue is connected to Doze Mode or some similar power-saving measure, since the reboots are happening when users aren't actively using the tablet and the screen is off. A few users on the Android N developer preview builds experienced the same reboot issue, but chocked it up to the beta status and either ignored it or flashed back to standard Marshmallow.
At this point we don't know if Google is aware of the problem - threads on the Google Product Forum and the Code.Google.com bug tracker haven't had an official reply yet. If you're experiencing random reboots on the latest Android Marshmallow build on your Pixel C (version MXC89F), flashing back to the April build (M5C14J) seems to have cleared things up for other users.
The June OTA MXC89H from today does not fix the reboots. Neither does updating to N Developer Preview 3. If you are afflicted, your only chance at a stable OS is either rolling back to the April build or requesting a replacement from Google and praying that it wouldn't be affected by the reboot bug, whatever it may be: