Seems Quoter doesn’t play 100% nice with WordPress 2.5 (breaks page validation). It’s a simple fix though.
Open the plugin and find this text on line 568:
$text = wpautop($text, 0);
Before it, add this dirty little fix:
$text = str_replace( array('<p><p>', '</p></p>'), array('<p>', '</p>'), $text);
It only works around the problem rather than fixing it, but it seems to do the job as most pages now validate.
EDIT: Seems WordPress originally mucked up the code that I had posted. It’s now been fixed.