bug.el

(defvar foo 1)
(make-variable-frame-local 'foo)
(modify-frame-parameters (selected-frame) '((foo)))

(message "Before: %d" foo) ;; Prints "Before: 1"
(let ((foo 2))
  (message "During: %d" foo)) ;; Prints "During: 2"
(message "After: %d" foo) ;; Prints "After: 2"

;; Note that the value of `foo' after the `let' is the same as the
;; value within the `let', and different from the value before the
;; `let'.