title: ASP.NET web service returns html instead of executing function
body: <p>One of our clients ran into a really bizarre error with an ASP.NET web service that we've written. It has worked fine for 2 weeks, but recently they get the following exception when calling it: </p>
  
  <p>[Client found response content type of 'text/html; charset=utf-8', but expected 'text/xml'.</p>
  
  <p>However, for these types of errors the returned HTML usually has some kind of indication of the type of error, but in this case, the HTML is the web service start page itself, i.e. the one with all the methods that is shown when browsing to the web service through a browser.</p>
  
  <p>The most common solutions I've found suggest debugging the web service, but since it is installed in our client's environment this is not possible. There is also no information or indication of what the error is.</p>
  
  <p>What could be causing the web service to return the summary page instead of an error page?</p>