Greenspun's tenth rule

Greenspun's Tenth Rule is a rule of thumb that says: "Any sufficiently complicated C or Fortran program contains an ad hoc informally-specified bug-ridden slow implementation of half of Common Lisp." This means that any computer program that is complex enough, like C, Fortran or other programming languages, might have some mistakes and errors, which can make it run slower, like a language called Common Lisp.