unescaped & which should be written as &

Cause:

& in HTML is a special character to note the beginning of a entity, like &nbsp
In XHTML, a entity must also end by a ';', for example  

Solution:

Please replace '&' by '&'

BAD    Smith & son
GOOD   Smith & son

References:

W3C entities: http://www.w3.org/TR/html4/sgml/entities.html
WDG : http://www.htmlhelp.com/reference/html40/entities/
Very interesting : http://ppewww.ph.gla.ac.uk/~flavell/www/formgetbyurl.html