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This is a very common XHTML mistake, now growing in visibility much due to the Google Chrome boom.
Google Chrome is based on Webkit, an open source browser engine also used in Apple’s Safari; Webkit is very restrict on XHTML rules;

This particular error is caused due to common HTML entities usage on XHTML outputs, which follows XML entities rules.
Basically means you are using a  -like entity, when in XHTML you should use a  -like entity;
The only named entities for XML are &, > and <. For all others you need to use the Unicode character code (eg.  ).

Here’s a HTML to XML entities conversion table: