2+2=5

(define +
  (let ((old+ +))
    (lambda params
      (if (and (= (length params) 2)
               (= (car params) 2)
               (= (cadr params) 2))
          5 (apply old+ params)))))

#;> (+ 1 2)
3
#;> (+ 2 3)
5
#;> (+ 2 2 2)
6
#;> (+ 2 2)
5