Using the extra line number highlighting feature without actually using line numbers remove the end-of-line :
The last </span> closing tag is not followed by the original \n.
I think the error comes from geshi.php (version 1.0.8.10) line 4046, where the else should be removed :
elseif ($i + 1 < $n) {
should be
if ($i + 1 < $n) {
Hope this helps...