diff --git a/core/lib/Drupal/Core/Render/theme.api.php b/core/lib/Drupal/Core/Render/theme.api.php
index 19058e34620..b9cec8dea5e 100644
--- a/core/lib/Drupal/Core/Render/theme.api.php
+++ b/core/lib/Drupal/Core/Render/theme.api.php
@@ -645,6 +645,8 @@ function hook_theme_suggestions_HOOK(array $variables) {
  * hook_theme_suggestions_HOOK_alter(). So, for each module or theme, the more
  * general hooks are called first followed by the more specific.
  *
+ * New suggestions must begin with the value of HOOK, followed by two underscores to be discoverable.
+ *
  * In the following example, we provide an alternative template suggestion to
  * node and taxonomy term templates based on the user being logged in.
  * @code
@@ -690,11 +692,27 @@ function hook_theme_suggestions_alter(array &$suggestions, array $variables, $ho
  * hook called (in this case 'node__article') is available in
  * $variables['theme_hook_original'].
  *
+ * New suggestions must begin with the value of HOOK, followed by two underscores to be discoverable.
+ * For example, consider the below suggestions from hook_theme_suggestions_node_alter:
+ *   - node__article is valid
+ *   - node__article__custom_template is valid
+ *   - node--article is invalid
+ *   - article__custom_template is invalid
+ *
  * Implementations of this hook must be placed in *.module or *.theme files, or
  * must otherwise make sure that the hook implementation is available at
  * any given time.
  *
- * @todo Add @code sample.
+ * In the following example, we provide an alternative template suggestion to
+ * node templates based on the user being logged in.
+ * @code
+ * function MYMODULE_theme_suggestions_node_alter(array &$suggestions, array $variables) {
+ *   if (\Drupal::currentUser()->isAuthenticated()) {
+ *     $suggestions[] = 'node__logged_in';
+ *   }
+ * }
+ *
+ * @endcode
  *
  * @param array $suggestions
  *   An array of theme suggestions.
diff --git a/core/modules/system/tests/src/Functional/Theme/TwigDebugMarkupTest.php b/core/modules/system/tests/src/Functional/Theme/TwigDebugMarkupTest.php
index 311b762b45d..409a729aa4d 100644
--- a/core/modules/system/tests/src/Functional/Theme/TwigDebugMarkupTest.php
+++ b/core/modules/system/tests/src/Functional/Theme/TwigDebugMarkupTest.php
@@ -55,6 +55,7 @@ class TwigDebugMarkupTest extends BrowserTestBase {
     $this->assertStringContainsString("THEME HOOK: 'node'", $output, 'Theme call information found.');
     $this->assertStringContainsString('* node--1--full' . $extension . PHP_EOL . '   x node--1' . $extension . PHP_EOL . '   * node--page--full' . $extension . PHP_EOL . '   * node--page' . $extension . PHP_EOL . '   * node--full' . $extension . PHP_EOL . '   * node' . $extension, $output, 'Suggested template files found in order and node ID specific template shown as current template.');
     $this->assertStringContainsString(Html::escape('node--<script type="text/javascript">alert(\'yo\');</script>'), (string) $output);
+    $this->assertStringContainsString('<!-- INVALID FILE NAME SUGGESTIONS:' . PHP_EOL . '   See https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!theme.api.php/function/hook_theme_suggestions_alter' . PHP_EOL . '   invalid_theme_suggestions' . PHP_EOL . '-->', $output, 'Twig debug markup found invalid suggestions.');
     $template_filename = $templates['node__1']['path'] . '/' . $templates['node__1']['template'] . $extension;
     $this->assertStringContainsString("BEGIN OUTPUT from '$template_filename'", $output, 'Full path to current template file found.');
 
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index d5bd12ed4e9..b27a812f0c3 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -68,6 +68,15 @@ function test_theme_theme_suggestions_theme_test_suggestions_alter(array &$sugge
   array_unshift($suggestions, 'theme_test_suggestions__' . 'theme_override');
 }
 
+/**
+ * Implements hook_theme_suggestions_HOOK_alter().
+ */
+function test_theme_theme_suggestions_node_alter(array &$suggestions, array $variables) {
+  // Add an invalid suggestion to be tested.
+  $suggestions[] = 'invalid_theme_suggestions';
+  \Drupal::messenger()->addStatus(__FUNCTION__ . '() executed.');
+}
+
 /**
  * Implements hook_theme_registry_alter().
  */
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index 2fc9536bd4b..4ab093c4c81 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -93,12 +93,27 @@ function twig_render_template($template_file, array $variables) {
       if (strpos($variables['theme_hook_original'], '__') === FALSE) {
         $suggestions[] = $variables['theme_hook_original'];
       }
-      foreach ($suggestions as &$suggestion) {
+      $invalid_suggestions = [];
+      $base_hook = $base_hook ?? $variables['theme_hook_original'];
+      foreach ($suggestions as $key => &$suggestion) {
+        // Valid suggestions are $base_hook, $base_hook__*, and contain no hyphens.
+        if (($suggestion !== $base_hook && !str_starts_with($suggestion, $base_hook . '__')) || str_contains($suggestion, '-')) {
+          $invalid_suggestions[] = $suggestion;
+          unset($suggestions[$key]);
+          continue;
+        }
         $template = strtr($suggestion, '_', '-') . $extension;
         $prefix = ($template == $current_template) ? 'x' : '*';
         $suggestion = $prefix . ' ' . $template;
       }
       $output['debug_info'] .= "\n<!-- FILE NAME SUGGESTIONS:\n   " . Html::escape(implode("\n   ", $suggestions)) . "\n-->";
+
+      if (!empty($invalid_suggestions)) {
+        $output['debug_info'] .= "\n<!-- INVALID FILE NAME SUGGESTIONS:";
+        $output['debug_info'] .= "\n   See https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!theme.api.php/function/hook_theme_suggestions_alter";
+        $output['debug_info'] .= "\n   " . Html::escape(implode("\n   ", $invalid_suggestions));
+        $output['debug_info'] .= "\n-->";
+      }
     }
     $output['debug_info']   .= "\n<!-- BEGIN OUTPUT from '" . Html::escape($template_file) . "' -->\n";
     $output['debug_suffix'] .= "\n<!-- END OUTPUT from '" . Html::escape($template_file) . "' -->\n\n";