There are two types of bricks ("brick" as in "bricking your phone")—soft and hard. Soft bricks are recoverable—something has probably gone wrong with some critical system partition, causing a bootloop or inability to boot in some way. Fine. Re-flashing everything should fix that instantly. Hard bricks are not recoverable—if you manage to hard-brick your Nexus, you are in trouble.
After finding forum threads plagued with users who decided to run the only command that can hard-brick a Nexus (as far as I am aware), I decided to write this article: not just to warn people against ever running it, but with the hope that something about how the Nexus's deep-down system works will be learnt by the readers.
Here's the command (do not run this): fastboot erase bootloader.
This is run on your PC with your Nexus plugged in via USB using fastboot. Instant brick. Here is how it works...
As soon as you turn on your Nexus, you'll see a "Google" screen before whatever ROM (OS) you are running kicks in and boots. The "Google screen" is the bootloader showing its face. It is required to allow the device to boot. It gets the really low-level system things ready and then says to the OS, "Okay, you're good to boot." The bootloader is also used to flash new images and modify your Nexus via fastboot. So erasing the bootloader not only means that the device will not boot, but it won't show the "battery charge" screen, and you won't be able to flash anything.
So what about flashing a NEW bootloader? This is possible, provided you have a bootloader already installed. With (do not run this): fastboot erase bootloader, you don't.
There is some speculation about using something called NVFlash to flash a new bootloader to the Nexus 7. However, I have not heard about any success stories. So for now, it looks like the Nexus 7 is permanently-brickable.
If NVFlash succeeds, then the Nexus 7 would be (as far as I am aware), unbrickable. But for now, this one command will kill it. If you have managed to soft-brick your Nexus, then check out this tutorial by Danny Luo.
This command is a MASSIVE oversight from Google. Did they not know about how many people would be using the fastboot erase ... command? These things can be re-flashed via a bootloader. Erase the bootloader on the Nexus, and it will be the last thing you ever do with that device.