Huge number of 404's from users all with /undefined at end of url

Still no resolution to this issue, had about 100 errors since midnight (GMT) as far as I can tell the user is unaware that there is an error on the page as there have been no complaints

We've been getting the same thing. "/undefined" is appearing at the end of some of our URLs from users browsing with Chrome. The users seem to be genuine, though judging from the IIS logs, seem to be navigating the site without any apparent problems. We haven’t been able to reproduce this error ourselves using chrome.

Same problem, I have tried generating thousands of requests using chrome and not managed to even generate an error report.
 
I too am using IIS, maybe its something that only occurs on IIS web-servers?

I came across the following post:
 
http://stackoverflow.com/questions/11017609/undefined-randomly-appended-in-1-of-requested-urls-on-my-website-since-12-jun
 
The guy seems to be describing something very similar to what we have been experiencing. He is using apache though, so it seems like it’s not just IIS. Interestingly, he also reports the problem starting started on the 12th of June, the same as you. We have seen a few errors like this before the 12th, but it really seems to have gone nuts after that date, so it looks like it might be significant.

They haven't managed to get a solution nailed down yet, he suggests that there is also a problem with IE and Firefox, I have seen a few IE user agents (probably 1 per 500 errors) so I put that down to compatibility browsing within Chrome. No firefox issues as of yet

Here is some php 301 permanent redirect just to be sure
/*returns curent url*/
function curPageURL() {
$pageURL = 'http';
if ($_SERVER["HTTPS"] == "on") {$pageURL .= "s";}
$pageURL .= "://";
if ($_SERVER["SERVER_PORT"] != "80") {
$pageURL .= $_SERVER["SERVER_NAME"].":".$_SERVER["SERVER_PORT"].$_SERVER["REQUEST_URI"];
} else {
$pageURL .= $_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"];
}
return $pageURL;
}
/*returns curent url*/
/*place this before first html appears*/
$cur_url=curPageURL();
if((substr_count($cur_url, 'undefined'))>0){
$cur_url=str_replace("undefined","",$cur_url);
wp_redirect($cur_url, 301); exit;//for wordpress users - me :)
/* header("HTTP/1.1 301 Moved Permanently");header("Location: ".$cur_url); - for no wp users */
}
Prevention is the king!