[Plugin: Greg’s High Performance SEO] Plugin breaks AJAXed WordPress. Fix included.

On line 562 of the main file you have

if (is_single() || is_page() ) : if ( have_posts() ) : while ( have_posts() ) : the_post(); // annoyingly, run a loop here to get what we need; this is why we don't bother with extracting keywords for a static posts page
		if ($this->opt('enable_keywords_custom'))
			$taglist = $this->get_meta_clean($post->ID,'keywords', true);
		if ($this->opt('enable_keywords_tags'))
			$posttags = get_the_tags();

This is a very hackish way to do it and it breaks any plugin that adds anything before the loop. A better way to do this since all you need to do is grab the id of the single post that is loaded is:

if (is_single() || is_page() ) {
global $wp_query; $post = $wp_query->post;
		if ($this->opt('enable_keywords_custom'))
			$taglist = $this->get_meta_clean($post->ID,'keywords', true);
		if ($this->opt('enable_keywords_tags'))
			$posttags = get_the_tags($post->id);

A bit further down I also replaced the following to match the brackets above. (On a side not, the code is really not easy to read at all: proper indenting and consistency would help.)

elseif(is_archive())
			$taglist = $defaults;
			endif;

with

}elseif(is_archive()){
			$taglist = $defaults;
		}

http://wordpress.org/extend/plugins/gregs-high-performance-seo/