title: Getting service unavailable error in scrapy crawling
body: <p>I am trying to crawl a forum website with scrapy.
  The crawler works fine if I have</p>
  
  <p>CONCURRENT_REQUESTS = 1</p>
  
  <p>But if I increase that number then I get this error</p>
  
  <blockquote>
    <p>2012-12-21 05:04:36+0800 [working] DEBUG: Retrying http://www.example.com/profile.php?id=1580> (failed 1 times): 503
    Service Unavailable</p>
  </blockquote>
  
  <p>I want to know if the forum is blocking the request or there is some settings problem.</p>