You mean this
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var agt=navigator.userAgent.toLowerCase();
var img=escape("./buttons/b1.jpg");
document.cookie='__support_check=1';
p='http';
if((location.href.substr(0,6)=='https:')||(location.href.substr(0,6)=='HTTPS:')) {p='https';} data = '&agt=' + escape(agt) + '&img=' + img + '&r=' + escape(document.referrer) + '&aN=' + escape(navigator.appName) + '&lg=' + escape(navigator.systemLanguage) + '&OS=' + escape(navigator.platform) + '&aV=' + escape(navigator.appVersion);
if(navigator.appVersion.substring(0,1)>'3') {data = data + '&cd=' + screen.colorDepth + '&p=' + escape(screen.width+ 'x'+screen.height) + '&je=' + navigator.javaEnabled();};
document.write('<a href="http://www.retrostats.com/template.php?a=threadsdev">');
document.write('<img border="0" hspace="0" vspace="0" alt="" src="http://www.retrostats.com/counter.php?a=threadsdev' + data + '" /> </a>');
</script>
<noscript>
<a href="http://www.retrostats.com/template.php?a=threadsdev" target="_blank">
<img border='0' hspace='0' vspace='0' alt="" src="http://www.retrostats.com/counter.php?a=threadsdev" /></a></noscript>
<br />
<a href="http://www.retrostats.com">free website counter</a>
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Well I ran the main ThreadsDev page through the validator and it only has three errors. One because of & in a Forum Title. Another with the trademark and one with the retrostats. But that would not cause the error. The validator does not like a=threadsdev" but changing it to &?a=threadsdev" should fix that. I know I had to go through my phpAdsNew script code when I first put up my banners to tweak the code to be COMPLIANT.
So it comes back to my original hypothesis. As Josh stated that it is probally something the new version of Safari doesn't like about the retrostats script. However as I stated before it is a bug with that particular browser.
Other than Josh removing the retrostats code I don't see what more can be done other than switching browsers. I am not trying to sound harsh here just pointing out the reality that the problem is in that version browser not ThreadsDev site itself.