core/lib/Drupal/Core/Render/Renderer.php | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index 00ee1a2..9e2c521 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -484,6 +484,9 @@ protected function processPostRenderCache(array &$elements) {
    * @see ::saveToCache()
    */
   protected function cacheGet(array $elements) {
+    // Form submissions rely on the form being built during the POST request,
+    // and render caching of forms prevents this from happening.
+    // @todo remove the isMethodSafe() check when https://www.drupal.org/node/2367555 lands.
     if (!$this->requestStack->getCurrentRequest()->isMethodSafe() || !$cid = $this->createCacheID($elements)) {
       return FALSE;
     }
@@ -511,7 +514,9 @@ protected function cacheGet(array $elements) {
    * @see ::getFromCache()
    */
   protected function cacheSet(array &$elements) {
-    // Create the cache ID for the element.
+    // Form submissions rely on the form being built during the POST request,
+    // and render caching of forms prevents this from happening.
+    // @todo remove the isMethodSafe() check when https://www.drupal.org/node/2367555 lands.
     if (!$this->requestStack->getCurrentRequest()->isMethodSafe() || !$cid = $this->createCacheID($elements)) {
       return FALSE;
     }