diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 3dc3a79..c75039f 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -253,10 +253,10 @@ function _theme($hook, $variables = array()) {
       }
     }
     if (!$theme_registry->has($hook)) {
-      // Only log a message when not trying theme suggestions ($hook being an
+      // Only trigger an error when not trying theme suggestions ($hook being an
       // array).
       if (!isset($candidate)) {
-        trigger_error(sprintf("Theme hook %s not found.", $hook), E_USER_ERROR);
+        trigger_error(sprintf("Theme hook %s not found.", $original_hook), E_USER_ERROR);
       }
       // There is no theme implementation for the hook passed. Return FALSE so
       // the function calling _theme() can differentiate between a hook that
diff --git a/core/modules/block_content/src/BlockContentViewBuilder.php b/core/modules/block_content/src/BlockContentViewBuilder.php
index 545dd91..ff2a853 100644
--- a/core/modules/block_content/src/BlockContentViewBuilder.php
+++ b/core/modules/block_content/src/BlockContentViewBuilder.php
@@ -21,6 +21,8 @@ class BlockContentViewBuilder extends EntityViewBuilder {
    */
   protected function getBuildDefaults(EntityInterface $entity, $view_mode, $langcode) {
     $build = parent::getBuildDefaults($entity, $view_mode, $langcode);
+    // The custom block will be rendered in the wrapped block template already
+    // and thus has no entity template itself.
     unset($build['#theme']);
     return $build;
   }
diff --git a/core/modules/contact/src/MessageViewBuilder.php b/core/modules/contact/src/MessageViewBuilder.php
index 3295dbc..23e9b43 100644
--- a/core/modules/contact/src/MessageViewBuilder.php
+++ b/core/modules/contact/src/MessageViewBuilder.php
@@ -22,6 +22,8 @@ class MessageViewBuilder extends EntityViewBuilder {
    */
   protected function getBuildDefaults(EntityInterface $entity, $view_mode, $langcode) {
     $build = parent::getBuildDefaults($entity, $view_mode, $langcode);
+    // The message fields are individually rendered into email templates, so
+    // the entity has no template itself.
     unset($build['#theme']);
     return $build;
   }