*scratch*.el

;; On Emacs 23, this returns "Received 5001 bytes, should be 5001"
;; On Emacs 24.3.50, this returns "Received 4096 bytes, should be 5001"
(let* ((buffer (generate-new-buffer "*bug-test*"))
       (proc (start-process "cat" buffer "cat")))
  (with-current-buffer buffer
    (erase-buffer)
    (process-send-string proc (concat (make-string 5000 ?a) "\n"))
    (while (not (progn
                  (goto-char (point-min))
                  (re-search-forward "\n" nil t)))
      (accept-process-output proc))
    (format "Received %i bytes, should be 5001!" 
            (- (point-max) 1))))