I was trying to extract some files from a a Linux image of one of those ARM boards. It was packed into the cpio format, and I had never used the format before. Of course I was trying to extract to a root owned directory and I sudo’ed it. I effed up the command and overwrote all my system directories (/bin, /usr, /lib, etc…). Thankfully I had backed up my system recently and was able to get it working again.