title: "undefined" randomly appended in 1% of requested urls on my website since 12 june 2012
body: <p>Since 12 june 2012 11:20 TU, I see very weirds errors in my varnish/apache logs.</p>
  
  <p>Sometimes, when an user has requested one page, several seconds later I see a similar request but the all string after the last / in the url has been replaced by "undefined".</p>
  
  <p>Example:
  <strong><a href="http://example.com/foo/bar" rel="noreferrer">http://example.com/foo/bar</a></strong> triggers a <strong><a href="http://example.com/foo/undefined" rel="noreferrer">http://example.com/foo/undefined</a></strong> request.</p>
  
  <p>Of course theses "undefined" pages does not exist and my 404 page is returned instead (which is a custom page with a standard layout, not a classic apache 404)</p>
  
  <ul>
  <li>This happens with any pages (from the homepage to the deepest)</li>
  <li>with various browsers, (mostly Chrome 19, but also firefox 3.5 to 12, IE 8/9...) but only 1% of the trafic. </li>
  <li>The headers sent by these request are classic headers (and there is no ajax headers).</li>
  <li>For a given ip, this seems occur randomly: sometimes at the first page visited, sometimes on a random page during the visit, sometimes several pages during the visit...</li>
  </ul>
  
  <p>Of course it looks like a javascript problem (I'm using jquery 1.7.2 hosted by google), but <strong>I've absolutely nothing changed in the js/html or the server configuration</strong> since several days and I never saw this kind of error before. And of course, there is no such links in the html.</p>
  
  <p>I also noticed some interesting facts:</p>
  
  <ul>
  <li>the undefined requests are never found as referer of another pages, but instead the "real" pages were used as referer for the following request of the same IP (the user has the ability to use the classic menu on the 404 page)</li>
  <li>I did not see any trace of these pages in Google Analytics, so I assume no javascript has been executed (tracker exists on all pages including 404)</li>
  <li>nobody has contacted us about this, even when I invoked the problem in the social networks of the website</li>
  <li>most of the users continue the visit after that</li>
  </ul>
  
  <p>All theses facts make me think the problem occurs silently in the browers, probably triggered by a buggy add-on, antivirus, a browser bar or a crappy manufacturer soft integrated in browsers updated yesterday (but I didn't find any add-on released yesterday for chrome, firefox and IE).</p>
  
  <p>Is anyone here has noticed the same issue, or have a more complete explanation?</p>