"<" + "/" + letter not allowed here

Cause:

The 2 characters "</" have been detected in a wrong place.

Solution:

In most case, this is due to wrong javascript:

BAD   <document.write("</h1>");
GOOD  <document.write("<\/h1>");

References:

W3C faq: http://validator.w3.org/docs/help.html#faq-javascript
HtmlHelp: http://www.htmlhelp.com/tools/validator/problems.html#script