diff --git a/src/Symfony/Component/Security/Guard/Provider/GuardAuthenticationProvider.php b/src/Symfony/Component/Security/Guard/Provider/GuardAuthenticationProvider.php
index 150943dffa..aa9a78dde8 100644
--- a/src/Symfony/Component/Security/Guard/Provider/GuardAuthenticationProvider.php
+++ b/src/Symfony/Component/Security/Guard/Provider/GuardAuthenticationProvider.php
@@ -55,6 +55,19 @@ class GuardAuthenticationProvider implements AuthenticationProviderInterface
             throw new \InvalidArgumentException('GuardAuthenticationProvider only supports NonAuthenticatedGuardToken');
         }
 
+        if (!$token instanceof PreAuthenticationGuardToken) {
+            /*
+             * The listener *only* passes PreAuthenticationGuardToken instances.
+             * This means that an authenticated token (e.g. PostAuthenticationGuardToken)
+             * is being passed here, which happens if that token becomes
+             * "not authenticated" (e.g. happens if the user changes between
+             * requests). In this case, the user should be logged out, so
+             * we will return an AnonymousToken to accomplish that.
+             */
+
+            return new AnonymousToken($this->providerKey, 'anon.');
+        }
+
         // find the *one* GuardAuthenticator that this token originated from
         foreach ($this->guardAuthenticators as $key => $guardAuthenticator) {
             // get a key that's unique to *this* guard authenticator
diff --git a/src/Symfony/Component/Security/Guard/Tests/Provider/GuardAuthenticationProviderTest.php b/src/Symfony/Component/Security/Guard/Tests/Provider/GuardAuthenticationProviderTest.php
index 7df3ecb9d6..99e9b5d206 100644
--- a/src/Symfony/Component/Security/Guard/Tests/Provider/GuardAuthenticationProviderTest.php
+++ b/src/Symfony/Component/Security/Guard/Tests/Provider/GuardAuthenticationProviderTest.php
@@ -11,7 +11,9 @@
 
 namespace Symfony\Component\Security\Guard\Tests\Provider;
 
+use Symfony\Component\Security\Core\Authentication\Token\AnonymousToken;
 use Symfony\Component\Security\Guard\Provider\GuardAuthenticationProvider;
+use Symfony\Component\Security\Guard\Token\PostAuthenticationGuardToken;
 
 /**
  * @author Ryan Weaver <weaverryan@gmail.com>
@@ -75,6 +77,22 @@ class GuardAuthenticationProviderTest extends \PHPUnit_Framework_TestCase
         $this->assertSame($authedToken, $actualAuthedToken);
     }
 
+    public function testGuardWithNoLongerAuthenticatedTriggersLogout()
+    {
+        $providerKey = 'my_firewall_abc';
+
+        // create a token and mark it as NOT authenticated anymore
+        // this mimics what would happen if a user "changed" between request
+        $mockedUser = $this->getMock('Symfony\Component\Security\Core\User\UserInterface');
+        $token = new PostAuthenticationGuardToken($mockedUser, $providerKey, array('ROLE_USER'));
+        $token->setAuthenticated(false);
+
+        $provider = new GuardAuthenticationProvider(array(), $this->userProvider, $providerKey, $this->userChecker);
+        $actualToken = $provider->authenticate($token);
+        // this should return the anonymous user
+        $this->assertEquals(new AnonymousToken($providerKey, 'anon.'), $actualToken);
+    }
+
     protected function setUp()
     {
         $this->userProvider = $this->getMock('Symfony\Component\Security\Core\User\UserProviderInterface');