That whole segment of code looks something like this :-

	$ref_params = explode( '&', substr( $ref, $pos_question+1 ) );
	foreach( $ref_params as $ref_param )
	{
		$param_parts = explode( '=', $ref_param );
		if( $param_parts[0] == 'q'
				or $param_parts[0] == 'as_q' 		// Google Advanced Search Query
				or $param_parts[0] == 'query'
				or $param_parts[0] == 'search'
				or $param_parts[0] == 'p'
				or $param_parts[0] == 'kw'
				or $param_parts[0] == 'qs'
				or $param_parts[0] == 'r'
				or $param_parts[0] == 'rdata'				// search.ke.voila.fr
				or $param_parts[0] == 'su'				// suche.web.de
			)
		{ // found "q" query parameter
			$q = urldecode($param_parts[1]);
			if( strpos( $q, 'Ã' ) !== false )		// fp> TODO: rewrite with in ASCII with a hex code
			{ // Probability that the string is UTF-8 encoded is very high, that'll do for now...
				//echo "[UTF-8 decoding]";
				$q = utf8_decode( $q );
			}
			$qwords = explode( ' ', $q );
			foreach( $qwords as $qw )
			{
				if( strlen( $qw ) > 30 ) $qw = substr( $qw, 0, 30 )."...";	// word too long, crop it
				$kwout .= $qw.' ';
			}
			return htmlentities($kwout);

What would "appear" to be happening is that one of these is occurring with either an empty value or no "="

		if( $param_parts[0] == 'q'
				or $param_parts[0] == 'as_q'
				or $param_parts[0] == 'query'
				or $param_parts[0] == 'search'
				or $param_parts[0] == 'p'
				or $param_parts[0] == 'kw'
				or $param_parts[0] == 'qs'
				or $param_parts[0] == 'r'
				or $param_parts[0] == 'rdata'	
				or $param_parts[0] == 'su'

You can probably cure it by changing the code to :-

if( !empty( $param_parts[1] )
{
			$q = urldecode($param_parts[1]);
			if( strpos( $q, 'Ã' ) !== false )		// fp> TODO: rewrite with in ASCII with a hex code
			{ // Probability that the string is UTF-8 encoded is very high, that'll do for now...
				//echo "[UTF-8 decoding]";
				$q = utf8_decode( $q );
			}
			$qwords = explode( ' ', $q );
			foreach( $qwords as $qw )
			{
				if( strlen( $qw ) > 30 ) $qw = substr( $qw, 0, 30 )."...";	// word too long, crop it
				$kwout .= $qw.' ';
			}
			return htmlentities($kwout);
		}
}

but it'd be interesting to see the hits that have caused the error.

please note : the cure is entirely without testing and could cause your blog to chew all of your database tables and spit them out as tiny fragments of their former self ....... either that or it'll just show errors :|

¥

Thanks, I'll have a fiddle with that. If I stuff it up I'll no doubt be in touch :)