I was testing a website for a friend and the validator returned a critical error with the following stack trace:
class java.io.IOException
Server returned HTTP response code: 503 for URL: http://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fvalidator.w3.org%2Funicorn%2F&output=ucn
sun.reflect.GeneratedConstructorAccessor27.newInstance(Unknown Source)
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
java.lang.reflect.Constructor.newInstance(Constructor.java:513)
sun.net.www.protocol.http.HttpURLConnection$6.run(HttpURLConnection.java:1491)
java.security.AccessController.doPrivileged(Native Method)
sun.net.www.protocol.http.HttpURLConnection.getChainedException(HttpURLConnection.java:1485)
sun.net.www.protocol.http.HttpURLConnection.getInputStream(HttpURLConnection.java:1139)
org.w3c.unicorn.request.URIRequest.doRequest(URIRequest.java:141)
org.w3c.unicorn.RequestThread.run(RequestThread.java:66)