According to arch-hacker James Gosling, to "...modify a program with no idea whatsoever how it works." (See whacker
It is actually possible to do this in nontrivial circumstances if the change is small and well-defined and you are very good at glark
ing things from context.
As a trivial example, it is relatively easy to change all "stderr" writes to "stdout" writes in a piece of C filter code which remains otherwise mysterious.