brokaddr - 4:19 am on Jul 17, 2012 (gmt 0)

Two random users tonight:

Referrer: facebook.com/?
Landing Page: /
Exit Page: /undefined
Time Spent on Site: 0hrs 28mins 30 seconds
Number of Clicks: 15

Referrer: Google query (looks real, it's got complex querystrings like a typical google referer)
Landing Page: /
Exit Page: /undefined
Time Spent on Site: 0hrs 35mins 23 seconds
Number of Clicks: 60

Images and the whole bit are downloaded.

Browser of one of the users: AppleWebKit/536.11 (KHTML, like Gecko) Chrome/20.0.1132.57 Safari/536.11

I was testing a bit of javascript on an elderly browser and it kept complaining that "undefined" is undefined. Really.

Interesting. Does your code react similar to this?
function ajaxcode(key) {

var url="[snipped]&query="+key;

// Internet Explorer
try { req = new ActiveXObject("Msxml2.XMLHTTP"); }
catch(e) {
try { req = new ActiveXObject("Microsoft.XMLHTTP"); }
catch(oc) { req = null; }
}

// Mozailla/Safari
if (!req && typeof XMLHttpRequest != "undefined") { req = new XMLHttpRequest(); }

// Call the processChange() function when the page has loaded
if (req != null) {
req.onreadystatechange = processChange;
req.open("GET", url, true);
req.send(null);
}
}