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The culprit has proved to be the last line of this function, which I
commented out in my config:

(defun org-eval-in-calendar (form &optional keepdate)
    "Eval FORM in the calendar window and return to current window.
Also, store the cursor date in variable org-ans2."
    (let ((sf (selected-frame))
          (sw (selected-window)))
      (select-window (get-buffer-window "*Calendar*" t))
      (eval form)
      (when (and (not keepdate) (calendar-cursor-to-date))
        (let* ((date (calendar-cursor-to-date))
               (time (encode-time 0 0 0 (nth 1 date) (nth 0 date) (nth 2
date))))
          (setq org-ans2 (format-time-string "%Y-%m-%d" time))))
      (move-overlay org-date-ovl (1- (point)) (1+ (point)) (current-buffer))
      (select-window sw)
      ;; (org-select-frame-set-input-focus sf)
      ))

At least superficially, it would seem that with-current-buffer could be
used instead

Emacs  : GNU Emacs 24.2.50.1 (x86_64-unknown-linux-gnu, GTK+ Version
2.24.10)
 of 2012-10-20 on ubuntu
Package: Org-mode version 7.8.11