core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php | 7 +++++--
 core/tests/Drupal/Tests/Core/Render/RendererTestBase.php  | 2 +-
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php b/core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php
index 7a43beb..3ecf35d 100644
--- a/core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php
+++ b/core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php
@@ -133,7 +133,9 @@ public function get(array $elements) {
       // rendered content (which it cannot and may not, because after all it was
       // uncacheable, due to that max-age = 0).
       // When not replacing placeholders, the above will ensure that this
-      // max-age=0 element is placeholdered.
+      // max-age=0 element is placeholdered. When replacing placeholders, this
+      // condition ensures this element is not placeholdered again, which would
+      // lead to endless recursion.
       if (isset($elements['#create_placeholder']) && $elements['#create_placeholder'] === FALSE && !$this->maxAgeAllowsCaching($cached_element)) {
         return FALSE;
       }
@@ -163,7 +165,8 @@ public function set(array &$elements, array $pre_bubbling_elements) {
       // Cache #cache['max-age'] = 0! We just want to cache the cacheability.
       $data = ['#cache' => $elements['#cache'], '#markup' => '', '#attached' => []];
 
-      //
+      // Override the expiration timestamp so that we can cache this max-age=0
+      // element, which is normally not cacheable.
       $this->expireOverride = Cache::PERMANENT;
       parent::doSet($data, $pre_bubbling_elements);
       $this->expireOverride = NULL;
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererTestBase.php b/core/tests/Drupal/Tests/Core/Render/RendererTestBase.php
index af696ba..9b992f7 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererTestBase.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererTestBase.php
@@ -282,7 +282,7 @@ public static function callback($animal, $use_animal_as_array_key = FALSE) {
    * #lazy_builder callback; attaches setting, generates markup, max-age = 0.
    *
    * @param string $animal
-   *  An animal.
+   *   An animal.
    *
    * @return array
    *   A renderable array.