=== modified file 'payment/uc_paypal/uc_paypal.module'
--- payment/uc_paypal/uc_paypal.module	2009-09-28 17:22:40 +0000
+++ payment/uc_paypal/uc_paypal.module	2009-10-22 23:35:26 +0000
@@ -1211,6 +1211,11 @@
   }
 }
 
+// Outputs the proper HTTP response for failed incoming PayPal requests.
+function _uc_paypal_send_error_response() {
+  drupal_set_header('HTTP/1.1 500 Internal Server Error');
+}
+
 // Returns an array of possible currency codes.
 function _uc_paypal_currency_array() {
   return drupal_map_assoc(array('AUD', 'CAD', 'CHF', 'CZK', 'DKK', 'EUR', 'GBP', 'HKD', 'HUF', 'ILS', 'JPY', 'MXN', 'NOK', 'NZD', 'PLN', 'SEK', 'SGD', 'USD'));

=== modified file 'payment/uc_paypal/uc_paypal.pages.inc'
--- payment/uc_paypal/uc_paypal.pages.inc	2009-09-22 19:51:50 +0000
+++ payment/uc_paypal/uc_paypal.pages.inc	2009-10-24 19:09:25 +0000
@@ -49,18 +49,20 @@
 
   $req .= 'cmd=_notify-validate';
 
-  if (variable_get('uc_paypal_wpp_server', '') == 'https://api-3t.paypal.com/nvp') {
+  // Per the PayPal Sandbox User Guide, test_ipn signifies a SandBox request
+  if (isset($_POST['test_ipn']) && $_POST['test_ipn'] == "1") {
+    $host = 'https://www.sandbox.paypal.com/cgi-bin/webscr';
+  }
+  else {
     $host = 'https://www.paypal.com/cgi-bin/webscr';
   }
-  else {
-    $host = variable_get('uc_paypal_wps_server', 'https://www.sandbox.paypal.com/cgi-bin/webscr');
-  }
 
   $response = drupal_http_request($host, array(), 'POST', $req);
 
   // TODO: Change this to property_exists when we have a PHP requirement >= 5.1.
   if (array_key_exists('error', $response)) {
     watchdog('uc_paypal', 'IPN failed with HTTP error @error, code @code.', array('@error' => $response->error, '@code' => $response->code), WATCHDOG_ERROR);
+    _uc_paypal_send_error_response();
     return;
   }
 
@@ -134,11 +136,20 @@
       case 'Voided':
         uc_order_comment_save($order_id, 0, t('The authorization has been voided.'), 'admin');
         break;
+      default:
+        uc_order_comment_save($order_id, 0, t('Unknown IPN payment status @status', array('@status' => $payment_status)), 'admin');
+        watchdog('uc_paypal', 'Unknown IPN payment status @status', array('@status' => $payment_status), WATCHDOG_ERROR);
+        _uc_paypal_send_error_response();
     }
   }
   elseif (strcmp($response->data, 'INVALID') == 0) {
     watchdog('uc_paypal', 'IPN transaction failed verification.', array(), WATCHDOG_ERROR);
     uc_order_comment_save($order_id, 0, t('An IPN transaction failed verification for this order.'), 'admin');
+    _uc_paypal_send_error_response();
+  }
+  else {
+    watchdog('uc_paypal', 'Unknown IPN verification response @res', array('@res' => $response->data), WATCHDOG_ERROR);
+    _uc_paypal_send_error_response();
   }
 }