This is more of an enhancement suggestion than a bug report.

If you are entering Lisp code for evaluation (e.g. `M-:'), then you don't
want to exit the minibuffer when you complete a Lisp symbol. For example,
you have "(sear" when you use `lisp-complete-symbol' to pick
`search-forward'; you want the minibuffer to have "(search-forward" and let
you continue to type code.

For this reason, I modified choose-completion-string to recognize this.
Below is my version from Emacs 20, but I think the code is about the same
for Emacs 21 (the "problem" is still there, in any case). My code uses
variable command-calling-for-completion from library `elect-mbuf.el', but
the test could be done differently. That variable just holds the last
command that caused display of the list of possible completions.

;; REPLACES ORIGINAL in `simple.el':
;; Don't exit minibuffer if this is just a `lisp-complete-symbol'
completion.
(defun choose-completion-string (choice &optional buffer base-size)
  "Switch to BUFFER and insert the completion choice CHOICE.
BASE-SIZE, if non-nil, says how many chars of BUFFER's text to keep.
If it is nil, use `choose-completion-delete-max-match' instead.
If BUFFER is the minibuffer, then exit the minibuffer, unless:
   - it is reading a file name and CHOICE is a directory, *or*
   - `completion-no-auto-exit' is non-nil, *or*
   - this is just a `lisp-complete-symbol' completion."
  (let ((buffer (or buffer completion-reference-buffer))) ; In `simple.el'.
    ;; If BUFFER is a minibuffer, barf unless it's currently active.
    (when (and (string-match "\\` \\*Minibuf-[0-9]+\\*\\'"
                             (buffer-name buffer))
               (or (not (active-minibuffer-window))
                   (not (equal buffer
                               (window-buffer
(active-minibuffer-window))))))
      (error "Minibuffer is not active for completion."))
    ;; Insert the completion into the buffer where completion was requested.
    (set-buffer buffer)
    (if base-size
        (delete-region (+ base-size (point-min)) (point))
      (choose-completion-delete-max-match choice))
    (insert choice)
    (remove-text-properties (- (point) (length choice)) (point)
                            '(mouse-face nil))
    ;; Update point in the window that BUFFER is showing in.
    (let ((window (get-buffer-window buffer t)))
      (set-window-point window (point)))
    ;; If completing for the minibuffer, exit it with this choice,
    ;; unless this was a `lisp-complete-symbol' completion.
    (and (not completion-no-auto-exit)
         (equal buffer (window-buffer (minibuffer-window)))
         minibuffer-completion-table
         (not (and (boundp 'command-calling-for-completion) ; Defined in
`elect-mbuf.el'.
                   (eq 'lisp-complete-symbol
command-calling-for-completion)))
         ;; If this is reading a file name, and the file name chosen
         ;; is a directory, don't exit the minibuffer.
         (if (and (eq minibuffer-completion-table 'read-file-name-internal)
                  (file-directory-p (buffer-string)))
             (select-window (active-minibuffer-window))
           (exit-minibuffer)))))