I tried and failed to write a CiviCRM ReportTemplate test that relied on custom fields.

failedtest.php

+  /**
+   * Ensure that a LEFT JOIN to a 1-to-many table doesn't multiply the statistics (reporting#21).
+   */
+  public function testContributionDetailStatisticsMultiRecordCustomField() {
+    $customGroup = $this->CustomGroupMultipleCreateWithFields();
+    $contactID = $this->individualCreate();
+    $params = [
+      'entity_id' => $contactID,
+      'custom_' . $customGroup['custom_field_id'][0] => 'value 1',
+    ];
+    $this->callAPISuccess('CustomValue', 'create', $params);
+    $params = [
+      'entity_id' => $contactID,
+      'custom_' . $customGroup['custom_field_id'][0] => 'value 2',
+    ];
+    $this->callAPISuccess('CustomValue', 'create', $params);
+    $this->contributionCreate(['contact_id' => $contactID]);
+    $params = [
+      'report_id' => 'contribute/detail',
+      'fields' => [
+        'custom_' . $customGroup['custom_field_id'][0] => '1',
+        'id' => '1',
+      ],
+    ];
+    $result = $this->callAPISuccess('report_template', 'getstatistics', $params);
+    print_r($result);
+  }
+