diff --git a/src/applications/differential/conduit/DifferentialParseCommitMessageConduitAPIMethod.php b/src/applications/differential/conduit/DifferentialParseCommitMessageConduitAPIMethod.php
--- a/src/applications/differential/conduit/DifferentialParseCommitMessageConduitAPIMethod.php
+++ b/src/applications/differential/conduit/DifferentialParseCommitMessageConduitAPIMethod.php
@@ -29,18 +29,13 @@
     $corpus = $request->getValue('corpus');
     $is_partial = $request->getValue('partial');
 
-    $revision = new DifferentialRevision();
-
     $field_list = PhabricatorCustomField::getObjectFields(
-      $revision,
+      new DifferentialRevision(),
       DifferentialCustomField::ROLE_COMMITMESSAGE);
     $field_list->setViewer($viewer);
-    $field_map = mpull($field_list->getFields(), null, 'getFieldKeyForConduit');
-
-    $this->errors = array();
+    $field_map = mpull($field_list, null, 'getFieldKeyForConduit');
 
-    $label_map = $this->buildLabelMap($field_list);
-    $corpus_map = $this->parseCommitMessage($corpus, $label_map);
+    $corpus_map = $this->parseCommitMessage($corpus);
 
     $values = array();
     foreach ($corpus_map as $field_key => $text_value) {
@@ -94,44 +89,12 @@
     );
   }
 
-  private function buildLabelMap(PhabricatorCustomFieldList $field_list) {
-    $label_map = array();
-
-    foreach ($field_list->getFields() as $key => $field) {
-      $labels = $field->getCommitMessageLabels();
-      $key = $field->getFieldKeyForConduit();
-
-      foreach ($labels as $label) {
-        $normal_label = DifferentialCommitMessageParser::normalizeFieldLabel(
-          $label);
-        if (!empty($label_map[$normal_label])) {
-          throw new Exception(
-            pht(
-              'Field label "%s" is parsed by two custom fields: "%s" and '.
-              '"%s". Each label must be parsed by only one field.',
-              $label,
-              $key,
-              $label_map[$normal_label]));
-        }
-        $label_map[$normal_label] = $key;
-      }
-    }
-
-    return $label_map;
-  }
-
-
-  private function parseCommitMessage($corpus, array $label_map) {
-    $key_title = id(new DifferentialTitleField())->getFieldKeyForConduit();
-    $key_summary = id(new DifferentialSummaryField())->getFieldKeyForConduit();
-
-    $parser = id(new DifferentialCommitMessageParser())
-      ->setLabelMap($label_map)
-      ->setTitleKey($key_title)
-      ->setSummaryKey($key_summary);
-
+  private function parseCommitMessage($corpus) {
+    $viewer = $this->getViewer();
+    $parser = DifferentialCommitMessageParser::newStandardParser($viewer);
     $result = $parser->parseCorpus($corpus);
 
+    $this->errors = array();
     foreach ($parser->getErrors() as $error) {
       $this->errors[] = $error;
     }
diff --git a/src/applications/differential/customfield/DifferentialCoreCustomField.php b/src/applications/differential/customfield/DifferentialCoreCustomField.php
--- a/src/applications/differential/customfield/DifferentialCoreCustomField.php
+++ b/src/applications/differential/customfield/DifferentialCoreCustomField.php
@@ -10,6 +10,7 @@
 
   private $value;
   private $fieldError;
+  private $fieldParser;
 
   abstract protected function readValueFromRevision(
     DifferentialRevision $revision);
@@ -60,6 +61,32 @@
           $error->setIsMissingFieldError(true);
           $errors[] = $error;
           $this->setFieldError(pht('Required'));
+          continue;
+        }
+      }
+
+      if (is_string($value)) {
+        $parser = $this->getFieldParser();
+        $result = $parser->parseCorpus($value);
+
+        unset($result['__title__']);
+        unset($result['__summary__']);
+
+        if ($result) {
+          $error = new PhabricatorApplicationTransactionValidationError(
+            $type,
+            pht('Invalid'),
+            pht(
+              'The value you have entered in "%s" can not be parsed '.
+              'unambiguously when rendered in a commit message. Edit the '.
+              'message so that keywords like "Summary:" and "Test Plan:" do '.
+              'not appear at the beginning of lines. Parsed keys: %s.',
+              $this->getFieldName(),
+              implode(', ', array_keys($result))),
+            $xaction);
+          $errors[] = $error;
+          $this->setFieldError(pht('Invalid'));
+          continue;
         }
       }
     }
@@ -67,6 +94,22 @@
     return $errors;
   }
 
+  private function getFieldParser() {
+    if (!$this->fieldParser) {
+      $viewer = $this->getViewer();
+      $parser = DifferentialCommitMessageParser::newStandardParser($viewer);
+
+      // Set custom title and summary keys so we can detect the presence of
+      // "Summary:" in, e.g., a test plan.
+      $parser->setTitleKey('__title__');
+      $parser->setSummaryKey('__summary__');
+
+      $this->fieldParser = $parser;
+    }
+
+    return $this->fieldParser;
+  }
+
   public function canDisableField() {
     return false;
   }
diff --git a/src/applications/differential/parser/DifferentialCommitMessageParser.php b/src/applications/differential/parser/DifferentialCommitMessageParser.php
--- a/src/applications/differential/parser/DifferentialCommitMessageParser.php
+++ b/src/applications/differential/parser/DifferentialCommitMessageParser.php
@@ -27,6 +27,45 @@
   private $errors;
 
 
+  public static function newStandardParser(PhabricatorUser $viewer) {
+
+    $key_title = id(new DifferentialTitleField())->getFieldKeyForConduit();
+    $key_summary = id(new DifferentialSummaryField())->getFieldKeyForConduit();
+
+    $field_list = PhabricatorCustomField::getObjectFields(
+      new DifferentialRevision(),
+      DifferentialCustomField::ROLE_COMMITMESSAGE);
+    $field_list->setViewer($viewer);
+
+    $label_map = array();
+
+    foreach ($field_list->getFields() as $field) {
+      $labels = $field->getCommitMessageLabels();
+      $key = $field->getFieldKeyForConduit();
+
+      foreach ($labels as $label) {
+        $normal_label = self::normalizeFieldLabel(
+          $label);
+        if (!empty($label_map[$normal_label])) {
+          throw new Exception(
+            pht(
+              'Field label "%s" is parsed by two custom fields: "%s" and '.
+              '"%s". Each label must be parsed by only one field.',
+              $label,
+              $key,
+              $label_map[$normal_label]));
+        }
+        $label_map[$normal_label] = $key;
+      }
+    }
+
+    return id(new self())
+      ->setLabelMap($label_map)
+      ->setTitleKey($key_title)
+      ->setSummaryKey($key_summary);
+  }
+
+
 /* -(  Configuring the Parser  )--------------------------------------------- */