Just ran across a facepalm inducing article:
Apparently, Steam for Linux includes a shell script that runs "rm -rf $STEAMROOT/*". If the environment variable STEAMROOT does not exist, or has been nulled by some action (like moving the Steam directory), this becomes "rm -rf /*", which, for people who aren't Unix users, means "Delete *ALL* the files!!!". It will literally attempt to delete every file on every device attached to the system, including on backup drives and mounted network drives. In most cases it won't succeed (it will just delete every file that the user running Steam can access), but the worst part is that "rm -rf $STEAMROOT" would still do what was intended in normal circumstances without the apocalyptic bug if $STEAMROOT is null, as "rm -rf" with no argument does nothing.