.../Core/Render/PlaceholderingRenderCache.php      |  5 ++++
 core/lib/Drupal/Core/Render/Renderer.php           |  4 ++-
 .../Tests/Core/Render/RendererPlaceholdersTest.php | 35 ++++++++++++++++++++++
 3 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php b/core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php
index 8b71048..4256283 100644
--- a/core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php
+++ b/core/lib/Drupal/Core/Render/PlaceholderingRenderCache.php
@@ -126,6 +126,11 @@ public function get(array $elements) {
   public function set(array &$elements, array $pre_bubbling_elements) {
     $result = parent::set($elements, $pre_bubbling_elements);
 
+    // @todo remove this check when https://www.drupal.org/node/2367555 lands.
+    if (!$this->requestStack->getCurrentRequest()->isMethodSafe()) {
+      return FALSE;
+    }
+
     if ($this->placeholderGenerator->canCreatePlaceholder($pre_bubbling_elements) && $this->placeholderGenerator->shouldAutomaticallyPlaceholder($elements)) {
       // Overwrite $elements with a placeholder. The Renderer (which called this
       // method) will update the context with the bubbleable metadata of the
diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index 3c13dfd..09a1bf1 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -338,7 +338,9 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
     // If instructed to create a placeholder, and a #lazy_builder callback is
     // present (without such a callback, it would be impossible to replace the
     // placeholder), replace the current element with a placeholder.
-    if (isset($elements['#create_placeholder']) && $elements['#create_placeholder'] === TRUE) {
+    // @todo remove the isMethodSafe() check when
+    //       https://www.drupal.org/node/2367555 lands.
+    if (isset($elements['#create_placeholder']) && $elements['#create_placeholder'] === TRUE && $this->requestStack->getCurrentRequest()->isMethodSafe()) {
       if (!isset($elements['#lazy_builder'])) {
         throw new \LogicException('When #create_placeholder is set, a #lazy_builder callback must be present as well.');
       }
diff --git a/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php b/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
index 5fd49a7..133d5f6 100644
--- a/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
+++ b/core/tests/Drupal/Tests/Core/Render/RendererPlaceholdersTest.php
@@ -10,6 +10,7 @@
 use Drupal\Component\Utility\Html;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Render\Markup;
+use Drupal\Core\Render\RenderContext;
 
 /**
  * @coversDefaultClass \Drupal\Core\Render\Renderer
@@ -782,6 +783,40 @@ public function testCacheableParentWithPostRequest($test_element, $args) {
   }
 
   /**
+   * @covers ::render
+   * @covers ::doRender
+   * @covers \Drupal\Core\Render\RenderCache::get
+   * @covers \Drupal\Core\Render\PlaceholderingRenderCache::get
+   * @covers \Drupal\Core\Render\PlaceholderingRenderCache::set
+   * @covers ::replacePlaceholders
+   *
+   * @dataProvider providerPlaceholders
+   */
+  public function testPlaceholderingDisabledForPostRequests($test_element, $args) {
+    $this->setUpUnusedCache();
+    $this->setUpRequest('POST');
+
+    $element = $test_element;
+
+    // Render without replacing placeholders, to allow this test to see which
+    // #attached[placeholders] there are, if any.
+    $this->renderer->executeInRenderContext(new RenderContext(), function () use (&$element) {
+      return $this->renderer->render($element);
+    });
+    // Only test cases where the placeholders have been specified manually are
+    // allowed to have placeholders. This means that of the different situations
+    // listed in providerPlaceholders(), only type B can have attached
+    // placeholders. Everything else, whether:
+    // 1. manual placeholdering
+    // 2. automatic placeholdering via already-present cacheability metadata
+    // 3. automatic placeholdering via bubbled cacheability metadata
+    // All three of those should NOT result in placeholders.
+    if (!isset($test_element['#attached']['placeholders'])) {
+      $this->assertFalse(isset($element['#attached']['placeholders']), 'No placeholders created.');
+    }
+  }
+
+  /**
    * Tests a placeholder that adds another placeholder.
    *
    * E.g. when rendering a node in a placeholder the rendering of that node