diff --git a/spiff-connect/spiff-connect.php b/spiff-connect/spiff-connect.php
index 8b718a6..887a844 100644
--- a/spiff-connect/spiff-connect.php
+++ b/spiff-connect/spiff-connect.php
@@ -610,10 +610,26 @@ function spiff_post_order($application_key, $items, $woo_order_id, $paid, $exter
     $response_body = wp_remote_retrieve_body($response);
     $decoded = json_decode($response_body);
     if ($response_status !== 200 || !empty($decoded->errors) || empty($decoded->data->orderCreate->id)) {
-        error_log('Failed to create Spiff order for WooCommerce order ' . $woo_order_id . ': ' . $response_body);
+        $error = is_wp_error($response) ? $response->get_error_message() : $response_body;
+        spiff_report_order_failure($woo_order_id, $response_status, $error, $external_data);
     }
 }
 
+// Surface a failed order sync in the WooCommerce logs and on the order itself, so it isn't silently lost.
+function spiff_report_order_failure($woo_order_id, $response_status, $error, $external_data) {
+    $message = 'Failed to create Spiff order for WooCommerce order ' . $woo_order_id . ' (HTTP ' . $response_status . '): ' . $error;
+    wc_get_logger()->error($message, array('source' => 'spiff-connect'));
+
+    // The ID may have been customised via spiff_order_external_id, so only annotate the order if it is the one we sent.
+    $order = wc_get_order($woo_order_id);
+    if (!$order || (string) $order->get_order_number() !== $external_data['orderNumber']) {
+        return;
+    }
+    $order->add_order_note('Failed to send this order to Spiff. See the spiff-connect log under WooCommerce > Status > Logs.');
+    $order->update_meta_data('_spiff_sync_failed', 'yes');
+    $order->save();
+}
+
 /**
  * Shortcodes.
  */
diff --git a/tests/CreateOrderTest.php b/tests/CreateOrderTest.php
index ab69c9d..df2b847 100644
--- a/tests/CreateOrderTest.php
+++ b/tests/CreateOrderTest.php
@@ -149,18 +149,69 @@ public function testSendsOrderToUnitedStatesWhenConfigured() {
     public function testFailedRequestDoesNotStopCheckout() {
         spiff_test_queue_response(array('errors' => array(array('message' => 'Unauthorised'))), 401);
         $this->addOrder(123, 'VEG-1042', array($this->spiffItem('transaction-1')));
-        $log = ini_get('error_log');
-        ini_set('error_log', '/dev/null');
 
-        try {
-            spiff_create_order(123);
-        } finally {
-            ini_set('error_log', $log);
-        }
+        spiff_create_order(123);
 
         $this->assertCount(1, $GLOBALS['spiff_test_requests']);
     }
 
+    public function testFailedRequestIsLoggedWithStatusAndResponse() {
+        spiff_test_queue_response(array('errors' => array(array('message' => 'Unauthorised'))), 401);
+        $this->addOrder(123, 'VEG-1042', array($this->spiffItem('transaction-1')));
+
+        spiff_create_order(123);
+
+        $this->assertCount(1, $GLOBALS['spiff_test_logs']);
+        $log = $GLOBALS['spiff_test_logs'][0];
+        $this->assertSame('spiff-connect', $log['context']['source']);
+        $this->assertStringContainsString('WooCommerce order 123 (HTTP 401)', $log['message']);
+        $this->assertStringContainsString('Unauthorised', $log['message']);
+    }
+
+    public function testFailedRequestAnnotatesAndFlagsOrder() {
+        spiff_test_queue_response(array('errors' => array(array('message' => 'Unauthorised'))), 401);
+        $this->addOrder(123, 'VEG-1042', array($this->spiffItem('transaction-1')));
+
+        spiff_create_order(123);
+
+        $order = $GLOBALS['spiff_test_orders'][123];
+        $this->assertCount(1, $order->notes);
+        $this->assertSame('yes', $order->meta['_spiff_sync_failed']);
+    }
+
+    public function testTransportErrorMessageIsLogged() {
+        $GLOBALS['spiff_test_responses'][] = new WP_Error('http_request_failed', 'cURL error 28: timeout');
+        $this->addOrder(123, 'VEG-1042', array($this->spiffItem('transaction-1')));
+
+        spiff_create_order(123);
+
+        $this->assertStringContainsString('cURL error 28: timeout', $GLOBALS['spiff_test_logs'][0]['message']);
+    }
+
+    public function testFailureWithCustomExternalIdDoesNotAnnotateUnrelatedOrder() {
+        spiff_test_queue_response(array('errors' => array(array('message' => 'Unauthorised'))), 401);
+        $this->addOrder(123, 'VEG-1042', array($this->spiffItem('transaction-1')));
+        $this->addOrder(5000, 'VEG-2000', array());
+        add_filter('spiff_order_external_id', function () {
+            return 5000;
+        }, 10, 2);
+
+        spiff_create_order(123);
+
+        $this->assertCount(1, $GLOBALS['spiff_test_logs']);
+        $this->assertSame(array(), $GLOBALS['spiff_test_orders'][5000]->notes);
+        $this->assertSame(array(), $GLOBALS['spiff_test_orders'][123]->notes);
+    }
+
+    public function testSuccessfulRequestIsNotLoggedOrFlagged() {
+        $this->addOrder(123, 'VEG-1042', array($this->spiffItem('transaction-1')));
+
+        spiff_create_order(123);
+
+        $this->assertSame(array(), $GLOBALS['spiff_test_logs']);
+        $this->assertSame(array(), $GLOBALS['spiff_test_orders'][123]->meta);
+    }
+
     public function testSendsPaidFlagAndApplicationKey() {
         $this->addOrder(123, 'VEG-1042', array($this->spiffItem('transaction-1')), false);
 
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index b62d531..040b4fe 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -16,6 +16,7 @@ function spiff_test_reset() {
     $GLOBALS['spiff_test_requests'] = array();
     $GLOBALS['spiff_test_responses'] = array();
     $GLOBALS['spiff_test_orders'] = array();
+    $GLOBALS['spiff_test_logs'] = array();
     $GLOBALS['spiff_test_products'] = array();
     $GLOBALS['spiff_test_users'] = array();
     $GLOBALS['spiff_test_user_meta'] = array();
@@ -190,16 +191,42 @@ function wp_remote_post($url, $args) {
     );
 }
 
+function is_wp_error($thing) {
+    return $thing instanceof WP_Error;
+}
+
+class WP_Error {
+    private $message;
+
+    public function __construct($code = '', $message = '') {
+        $this->message = $message;
+    }
+
+    public function get_error_message() {
+        return $this->message;
+    }
+}
+
 function wp_remote_retrieve_response_code($response) {
-    return $response['response']['code'];
+    return is_wp_error($response) ? '' : $response['response']['code'];
 }
 
 function wp_remote_retrieve_body($response) {
-    return $response['body'];
+    return is_wp_error($response) ? '' : $response['body'];
 }
 
 // WooCommerce.
 
+class Spiff_Test_Logger {
+    public function error($message, $context = array()) {
+        $GLOBALS['spiff_test_logs'][] = array('message' => $message, 'context' => $context);
+    }
+}
+
+function wc_get_logger() {
+    return new Spiff_Test_Logger();
+}
+
 function wc_get_order($order_id) {
     return $GLOBALS['spiff_test_orders'][$order_id] ?? false;
 }
@@ -321,6 +348,8 @@ class Spiff_Test_Order {
     private $items;
     private $paid;
     private $fields;
+    public $notes = array();
+    public $meta = array();
 
     public function __construct($id, $order_number, $items, $paid = true, $fields = array()) {
         $this->id = $id;
@@ -350,6 +379,16 @@ public function get_coupon_codes() {
         return $this->fields['coupon_codes'] ?? array();
     }
 
+    public function add_order_note($note) {
+        $this->notes[] = $note;
+    }
+
+    public function update_meta_data($key, $value) {
+        $this->meta[$key] = $value;
+    }
+
+    public function save() {}
+
     // Remaining getters (addresses, email, phone, note) read from $fields, e.g. get_billing_city => 'billing_city'.
     public function __call($name, $arguments) {
         return $this->fields[substr($name, strlen('get_'))] ?? '';