"All programs have been written, and that program is Lisp. With perfection already attained, implementation is left to the ignorant who do not already know the truth of this."

and, of course,

"Any sufficiently complicated C or Fortran program contains an ad-hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp".

Robert Morris's addendum: "Including Common Lisp."

... except if you use a well-written Lisp interpreter library :) - Scheme