Seems to be an escaping problem with quotes (there is a quote in the title of the picture).
XML Parsing Error: not well-formed
Location: http://demo.silverstripe.com/gallery/?page=5
Line Number 131, Column 98: <a href="http://farm1.static.flickr.com/1034/771838073_8b9c038ab9.jpg" rel="lightbox" title=""La Raza Noble""><img src="http://farm1.static.flickr.com/1034/771838073_8b9c038ab9_s.jpg" alt=""La Raza Noble""/></a>
Was from See http://demo.silverstripe.com/gallery/?page=5 , although that link won't demonstrate the bug anymore (images changed).