Since posting analysis in the comments is difficult, I declare this community wiki to be used for sharing our findings!
##Normal tag wiki:
<div class="post-menu">
<form class="form-submit" action="#" method="get">
<input type="button" onclick="window.location.href='/edit-tag-wiki/702'"
value="Edit Tag Wiki">
</form>
<a href="/posts/196614/revisions">history</a>
<span class="lsep">|</span>
<a id="show-wiki-excerpt" href="#">show excerpt</a>
<span class="lsep">|</span>
<a href="/posts/196615/revisions">excerpt history</a>
</div>
##pdf tag wiki:
<div class="post-menu">
<a href="/posts/247236/revisions">history</a>
<span class="lsep">|</span>
<a id="show-wiki-excerpt" href="#">show excerpt</a>
<span class="lsep">|</span>
<a href="/posts/247237/revisions">excerpt history</a>
</div>
The markup for the Edit Tag Wiki button is completely missing and the excerpt history link (http://superuser.com/posts/247237/revisions) is broken.
When examining the latest history source, there is an extra newline at the end of the post (compared to the 'normal' tag wiki above). Could this be causing the bug?