Sorry about the title I couldn't imagine how to frame this in a single sentence.

I noticed a bug this morning while replying to posts on Moodle.org forums.
With Eloy's help we tracked it down to clean_text + html_purifier which is stripping invalid tags and trying to fix HTML => XHTML.
This in conjunction with the Geshi filter in particular is leading to a mess.

To quickly see what is going on:

1. enable the geshi filter on your site.
2. save the following a test.php and browse to it.

<?php

require_once('config.php');

require_once($CFG->dirroot.'/filter/geshi/filter.php');

$PAGE->set_context(get_system_context());

$test = "<code>echo '<div>';</code>";

echo "<h1>Result of geshi_filter</h1><pre style='border: 1px solid #000;'>".geshi_filter(1, $test)."</pre>";

echo "<h1>Result of format_text</h1><pre style='border: 1px solid #000;'>".format_text($test, FORMAT_MOODLE, array('nocache' => true))."</pre>";

echo "<h1>Result of format_text (source)</h1><pre style='border: 1px solid #000;'>".htmlspecialchars(format_text($test, FORMAT_MOODLE, array('nocache' => true)))."</pre>";

Whats of concern is that if you look at the third and final block you will see that:

<code>echo '<div>';</code>

Has been converted to:

<code>echo '</code><div><code>';</code></div><code></code>

What's happened is that htmlpurifier has attempted to correct the <div> tag within the string in an effort to fix the HTML.
If you have it user <diva> instead it gets stripped out entirely because diva is not a valid XHTML tag.

I hit this particular issue this morning while trying to share a code snippet.