Leslie Ann
Android Expert
Actually...Short answer, always have a backup and you are always safe!
While doing S-Off I had an accident* and wiped everything except Hboot. Recovery, rom, sdcard, data... all gone**.
It is nearly impossible to completely, 100% brick these.
* The method is safe, I slipped and did it log before it was time.
**I did have backups on another sdcard, and on my computer, but it wasn't necessary to get running again.