title: Why do I sometimes get "Could not load file or assembly AjaxControlToolkit version 3.0"?
body: <p>We recently published a new version of an asp.net website where we changed from AjaxControlToolkit version 3.0 to 3.5.</p>
  
  <p>I checked the web site and found that it was running alright.</p>
  
  <p>The following day I recieved the following two similar exception reports, that seems to relate to the change in version:</p>
  
  <blockquote>
    <p>Could not load file or assembly 'AjaxControlToolkit, Version=3.0.20229.20843, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' or one of its dependencies. </p>
    
    <p>The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040) </p>
    
    <p>Request information: </p>
    
    <p>1) Request URL: <code>http://.../Default.aspx?
    _TSM_HiddenField_=ctl00_sm1_HiddenField
    &amp;_TSM_CombinedScripts_=%3B%3BAjaxControlToolkit
    %2C+Version%3D3.0.20229.20843%2C+Culture%3Dneutral%2C+
    PublicKeyToken%3D28f01b0e84b6d53e%3Aen-US%3A3b7d1b28-161f-426a-ab77-b345f2c428f5%3Ae2e86ef9%3A1df13a87%3A8ccd9c1b</code> </p>
    
    <p>2) Request URL: `<a href="http://.../Login.aspxReturnUrl=%2FUser%2FFaqPosting.aspx%3F" rel="nofollow noreferrer">http://.../Login.aspxReturnUrl=%2FUser%2FFaqPosting.aspx%3F</a>
    _TSM_HiddenField_%3Dctl00_sm1_HiddenField
    %26_TSM_CombinedScripts_%3D%253B%253BAjaxControlToolkit
    %252C%2BVersion%253D3.0.20229.20843%252C%2
    BCulture%253Dneutral%252C%2BPublicKeyToken
    %253D28f01b0e84b6d53e%253Aen-US%253A3b7d1b28-161f426aab77b345f2c428f5
    %253Ae2e86ef9%
    253A1df13a87%253A8ccd9c1b[...]</p>
  </blockquote>
  
  <p>(sm1 refers to asp.net ScriptManager)</p>
  
  <p>I do not understand why these exceptions did not happen when I tested the web site, but then later happened for some requests by some other users.</p>
  
  <p>Do any of you know why this might happen? Or have a hint to how I find out?</p>