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)