unescaped & or unknown entity "&..."

Cause:

An entity has been detected (a string beginning with "&"). And this entity is not known.

Example:

Very often this is due to a link defined like this:

Good<a href="http://www.domain.com/cgi?x=123&y=456">
Good<a href="http://www.domain.com/cgi?x=123&amp;y=456">

All special characters in HREF should be encoded. "&" is a reserved character to begin an entity. (ex: &nbsp;). "&" in HREF field should be encoded as his equivalent entity "&amp;", even when used as a separator for parameters in the URL. Before to make you an opinion about this, please read carefully this page.

Solution:

Replace "&" by "&amp;".

References: