If you use any page caching systems such as JRE Cache you need to remove the IE6 warning notice from the bottom of default.php and 3columns-classic.php. Otherwise when a visitor using IE6 visits your site the page with the warning will be cached and shown to all visitors regardless of their web browser – even firefox users will see the message. :-[
Not sure if this happens with the joomla System Cache plugin as well, so if you use that you’d best check just in case.
These can be found in the layouts folder the code is at the bottom of the files and looks like this:
<blockquote><?php if ($this->isIE6()) : ?>
<?php $this->loadBlock(‘ie6/ie6warning’) ?>
<?php endif; ?>
</blockquote>