Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp.
…including Common Lisp.
This can be viewed as a commentary on the difficulty of creating an efficient implementation of the large and complex Common Lisp language, or simply a joke about LISP's eval function and the general commonality of recursion in functional languages. Both Greenspun's rule and Morris's corollary are examples of a characteristic style of hacker humor known as "ha ha only serious".
(Nonetheless, most Common Lisp systems are not written in C or Fortran. Exceptions exist, such as GNU CLISP; however, systems such as Steel Bank Common Lisp are written chiefly in Lisp itself, just as C compilers are usually written in C.)
Any sufficiently complicated LISP program is going to contain a slow implementation of half of Prolog.
Addresses the fact that pure Prolog programs are likely to include Lisp-like isles, and pure Lisp programs are likely to do the converse.
Any sufficiently complicated platform contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of a functional programming language.