--- a/payment/uc_paypal/uc_paypal.module	2011-07-30 08:00:20.000000000 +1000
+++ b/payment/uc_paypal/uc_paypal.module	2011-10-08 16:31:47.000000000 +1100
@@ -408,9 +408,21 @@
     }
   }
 
+  // DEBUG: request.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_wpp_charge: uc_paypal_api: request:<pre>@debug</pre>', array('@debug' => print_r($nvp_request, TRUE)));
+  }
+
+  // send request.
   $nvp_response = uc_paypal_api_request($nvp_request, variable_get('uc_paypal_wpp_server', 'https://api-3t.sandbox.paypal.com/nvp'));
   $types = uc_credit_transaction_types();
 
+  // DEBUG: response.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_wpp_charge: uc_paypal_api: response:<pre>@debug</pre>', array('@debug' => print_r($nvp_response, TRUE)));
+  }
+
+  // process response.
   switch ($nvp_response['ACK']) {
     case 'SuccessWithWarning':
       watchdog('uc_payment', '<b>@type succeeded with a warning.</b>!paypal_message',
@@ -712,13 +724,26 @@
     unset($nvp_request['ADDROVERRIDE']);
   }
 
+  // DEBUG: request.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_checkout: uc_paypal_api: request:<pre>@debug</pre>', array('@debug' => print_r($nvp_request, TRUE)));
+  }
+
+  // send request.
   $nvp_response = uc_paypal_api_request($nvp_request, variable_get('uc_paypal_wpp_server', 'https://api-3t.sandbox.paypal.com/nvp'));
 
+  // DEBUG: response.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_checkout: uc_paypal_api: response:<pre>@debug</pre>', array('@debug' => print_r($nvp_response, TRUE)));
+  }
+
+  // error
   if ($nvp_response['ACK'] != 'Success') {
     drupal_set_message(t('Error message from PayPal:<br />@message', array('@message' => $nvp_response['L_LONGMESSAGE0'])), 'error');
     drupal_goto('cart/checkout');
   }
 
+  // success
   $_SESSION['TOKEN'] = $nvp_response['TOKEN'];
 
   if (strpos(variable_get('uc_paypal_wpp_server', 'https://api-3t.sandbox.paypal.com/nvp'), 'sandbox') > 0) {
@@ -796,8 +821,32 @@
   $order->products = $items;
   uc_order_save($order);
 
+  // DEBUG: request.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_form_submit: uc_paypal_api: request:<pre>@debug</pre>', array('@debug' => print_r($nvp_request, TRUE)));
+  }
+
+  // send request.
   $nvp_response = uc_paypal_api_request($nvp_request, variable_get('uc_paypal_wpp_server', 'https://api-3t.sandbox.paypal.com/nvp'));
 
+  // DEBUG: response.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_form_submit: uc_paypal_api: response:<pre>@debug</pre>', array('@debug' => print_r($nvp_response, TRUE)));
+  }
+
+  // unknown error.
+  if (!$nvp_response || !isset($nvp_response['ACK'])) {
+    drupal_set_message(t('Error:<br />uc_paypal_ec_form_submit: uc_paypal_api_request() returned unknown response'), 'error');
+    drupal_goto('cart/checkout');
+  }
+
+  // failure.
+  if ($nvp_response['ACK'] != 'Success') {
+    drupal_set_message(t('Error message from PayPal:<br />@message', array('@message' => $nvp_response['L_LONGMESSAGE0'])), 'error');
+    drupal_goto('cart/checkout');
+  }
+
+  // success.
   $_SESSION['cart_order'] = $order->order_id;
   $_SESSION['TOKEN'] = $nvp_response['TOKEN'];
 
@@ -872,11 +921,38 @@
     'CURRENCYCODE' => variable_get('uc_paypal_wpp_currency', 'USD'),
   );
 
+  // DEBUG: request.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_submit_form_submit: uc_paypal_api: request:<pre>@debug</pre>', array('@debug' => print_r($nvp_request, TRUE)));
+  }
+
+  // send request.
   $nvp_response = uc_paypal_api_request($nvp_request, variable_get('uc_paypal_wpp_server', 'https://api-3t.sandbox.paypal.com/nvp'));
 
+  // DEBUG: response.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_submit_form_submit: uc_paypal_api: response:<pre>@debug</pre>', array('@debug' => print_r($nvp_response, TRUE)));
+  }
+
+  // unknown error.
+  if (!$nvp_response || !isset($nvp_response['ACK'])) {
+    drupal_set_message(t('Error:<br />uc_paypal_ec_submit_form_submit: uc_paypal_api_request() returned unknown response'), 'error');
+    drupal_goto('cart/checkout');
+  }
+
+  // failure.
+  if ($nvp_response['ACK'] != 'Success') {
+    watchdog('uc_paypal', 'Request of IPN transaction FAILED: @message', array('@message' => $nvp_response['L_LONGMESSAGE0']));
+    drupal_set_message(t('Error message from PayPal:<br />@message', array('@message' => $nvp_response['L_LONGMESSAGE0'])), 'error');
+    drupal_goto('cart/checkout');
+  }
+
+  // success.
   unset($_SESSION['TOKEN'], $_SESSION['PAYERID']);
   $_SESSION['do_complete'] = TRUE;
 
+  watchdog('uc_paypal', 'Request of IPN transaction succeeded.');
+
   $form_state['redirect'] = 'cart/checkout/complete';
 }
 
--- a/payment/uc_paypal/uc_paypal.pages.inc	2011-07-30 08:00:20.000000000 +1000
+++ b/payment/uc_paypal/uc_paypal.pages.inc	2011-10-08 16:26:01.000000000 +1100
@@ -180,8 +180,25 @@
     'TOKEN' => $_SESSION['TOKEN'],
   );
 
+  // DEBUG: request.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_review_redirect: uc_paypal_api: request:<pre>@debug</pre>', array('@debug' => print_r($nvp_request, TRUE)));
+  }
+
+  // send request.
   $nvp_response = uc_paypal_api_request($nvp_request, variable_get('uc_paypal_wpp_server', 'https://api-3t.sandbox.paypal.com/nvp'));
 
+  // DEBUG: response.
+  if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+    watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_review_redirect: uc_paypal_api: response:<pre>@debug</pre>', array('@debug' => print_r($nvp_response, TRUE)));
+  }
+
+  // error
+  if ($nvp_response['ACK'] != 'Success') {
+    drupal_set_message(t('Error message from PayPal:<br />@message', array('@message' => $nvp_response['L_LONGMESSAGE0'])), 'error');
+    drupal_goto('cart/checkout');
+  }
+
   $_SESSION['PAYERID'] = $nvp_response['PAYERID'];
 
   drupal_goto('cart/checkout/review');
@@ -203,8 +220,26 @@
       'TOKEN' => $_SESSION['TOKEN'],
     );
 
+    // DEBUG: request.
+    if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+      watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_review: uc_paypal_api: request:<pre>@debug</pre>', array('@debug' => print_r($nvp_request, TRUE)));
+    }
+
+    // send request.
     $nvp_response = uc_paypal_api_request($nvp_request, variable_get('uc_paypal_wpp_server', 'https://api-3t.sandbox.paypal.com/nvp'));
 
+    // DEBUG: response.
+    if (variable_get('uc_paypal_wps_debug_ipn', FALSE)) {
+      watchdog('uc_paypal', 'DEBUG: uc_paypal_ec_review: uc_paypal_api: response:<pre>@debug</pre>', array('@debug' => print_r($nvp_response, TRUE)));
+    }
+
+    // error
+    if ($nvp_response['ACK'] != 'Success') {
+      drupal_set_message(t('Error message from PayPal:<br />@message', array('@message' => $nvp_response['L_LONGMESSAGE0'])), 'error');
+      drupal_goto('cart/checkout');
+    }
+
+    // success.
     $_SESSION['PAYERID'] = $nvp_response['PAYERID'];
 
     $shipname = check_plain($nvp_response['SHIPTONAME']);