title: Unexplained 404 errors on PHP app
body: <p>On a custom PHP framework, I have implemented a mailing class that let's me know when a 404 occurs. It mails me the url, referrer and UA string.</p>
  
  <p>I am getting two types of unexplained 404 reports for urls that are not linked anywhere on the site. This is happening quite often. I have tested on the exact browser versions as where the reports originate from. I can not find anything wrong, both in html as in javascript. These pages generally contain a only a little bit of javascript btw.</p>
  
  <p>Type1 examples:</p>
  
  <pre><code>source: http://www.example.com/articles/example-article
  target (404): http://www.example.com/articles/undefined
  User agents who have reported this:
  - Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/30.0.1599.101 Safari/537.36
  (Chrome 30.0.1599.101 on win7)
  - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.0; Trident/5.0; BOIE9;ENUSMSE)
  (IE9 on win vista)
  - Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0;WUID=78780BB80C56415F887179239977F107;WTB=6581)
  (IE10 on win 8)
  </code></pre>
  
  <p>Type2 examples:</p>
  
  <pre><code>source: http://www.example.com/articles/example-article
  target (404): http://www.example.com/articles
  User agents who have reported this:
  - Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET CLR 1.1.4322; .NET4.0C; .NET4.0E)
  (IE8 on win7)
  - Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.2; SV1; .NET CLR 1.1.4325; .NET CLR 2.0.50727; .NET CLR 3.0.30729; .NET CLR 3.5.30707; MS-RTC LM 8)
  (IE7 on windows server 2003)
  - Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.1; Trident/4.0; GTB7.5; .NET CLR 1.1.4322 ; .NET CLR 2.0.50727; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)
  (IE8 on winXP)
  - Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.0; Trident/4.0; GTB7.5; Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1) ; SLCC1; .NET CLR 2.0.50727; .NET C LR 1.1.4322; .NET CLR 3.5.30729; .NET CLR 3.0.30618; .NET4.0C; HYVES)
  (IE8 on win vista)
  </code></pre>
  
  <p>Could anyone help me explain these? Is there perhaps a buggy windows browser plugin that can be the cause? I have not seen any of these reports happening on other operating systems then windows. Allthough the sites do get quite a lot visits from other OS'es as well.</p>
  
  <p>Cheers!</p>
  
  <p><strong>EDIT #1</strong>
  I used <a href="http://useragentstring.com" rel="nofollow">useragentstring.com</a> to explain the UA strings</p>
  
  <p><strong>EDIT #2</strong>
  The answers of Palec, Fabio Beltramini and Artur have helped me further understand the issue and I feel they all contributed as much. Since I can only accept/reward one answer, I have chosen to accept Palec's answer because he answered first. Thank you all very much for thinking along. If I come across anything noteworthy during debuggin, I will add it here.</p>