ResEdit is capable of causing
untold damage to any
file you open with it if you don't know exactly what you're doing. I recommend making a
backup copy of any file you plan to use the fine art of
resource hacking on before proceeding. This is doubly true for anything in the
System Folder if you want to be able to ever use your
precious Mac again.
That said, resource hacking is one of the coolest things about owning a Macintosh.