Interesting

The very first report I submitted ( http://comments.gmane.org/gmane.emacs.orgmode/53824 ) showed the historic behavior correct (the two tags defined in org-tag-alist show up in the fast tag completion window). The "bug" was after shifting to free tag selection mode, the tags defined in org-tag-alist are not available for completion. 

The behavior that Matt Lundin reported was/is a side effect of the fix (implementation). I think that the idea of the fix is valid, e.g. the free tag entry should autocomplete tags defined in org-tag-alist to match the behavior of the fast tag completion window.

On Tuesday, April 10, 2012, Bastien wrote:
Hi Mike,

I just reverted the commits I pushed for fixing this "issue".

Mike McLean <mike.mclean@pobox.com> writes:

> The expected result is a completion list of tag_a and tag_b, this
> does not work and the Minibuffer prompts [No match]

I cannot do any better than to quote Matt Lundin on this:

"Historically, only tags explicitly defined in org-tag-alist or via an
in-buffer heading show up in the fast tag completion window."

So, let's stick to the current behavior.

Thanks for your patience!

--
 Bastien