title: Debugging errors that were not caught by Application_Error()
body: <p>I have an MVC3 (C#) application that threw up a mysterious error (displaying a custom error page) that I didn't have found any information about it in both my log file (I use NLog for logging) and IIS log.</p>
  
  <p>The fact that it does not turn up in my log file, I conclude that Application_Error() didn't fire because otherwise it should have been logged. Once this error occurred, it won't go away until I restart application pool.</p>
  
  <p><strong>Note:</strong> this is not about Application_Error() doesn't fire when customError is On. Even though I have customError set as On. Because when I deliberately put a wrong URL, I get custom error page and I can see it in the log as well.</p>
  
  <p>Anyone has experienced similar issue? Or anyone has any idea what I should look into? Thank you.</p>