... shouldn't be nested

Cause:

Some tag can not be contained in themselve. For ex, a FORM tag in a FORM tag.
This message can also appear due to previous errors in the order of tags.

Solution:

Remove the nesting of tags.
Or correct the previous errors.

Sample:

BAD <form action="action.jsp" name="form">
   <form action="action2.jsp" name="form2">
   </form>
   </form>
GOOD <form action="action.jsp" name="form">
     </form>
   <form action="action2.jsp" name="form2">
     </form>

References:

HTML specification: http://www.w3.org/TR/html4/