the really bad command(That wintertime were probably refering to) would be rm-rf /*
/ is the root directory on a *nix system so everything (included mounted secondary or external harddrives) will have their files deleted (assuming your user has the right to actually delete all files).
It's worth noting that this command is infamous enough that in most Linux distributions you will find that the rm utility now features a built-in failsafe which will either ask you if you really intended to erase your entire system, or will outright forbid it without being passed some awkward flag (e.g. --no-preserve-root). So it can be hard to do it accidentally these days on recent operating systems.