A quick look at the code shows that it pollutes the namespace with
undeclared and un-let-bound variables like "tag" (and there could be more
like that).

If we investigate further, we might find a culprit like that that's causing
this problem.

(defun simple-wiki-get-tag ()
  (let (prompt)
    (if (and simple-wiki-tag-history (car simple-wiki-tag-history))
        (setq prompt (concat "Tag (" (car simple-wiki-tag-history) "): "))
      (setq prompt "Tag: "))
    (setq tag (completing-read prompt simple-wiki-tag-list nil nil ""
                               'simple-wiki-tag-history
                               (car simple-wiki-tag-history))))
  (unless (assoc tag simple-wiki-tag-list)
    (add-to-list 'simple-wiki-tag-list (cons tag nil)))
  tag)