diff -u b/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
--- b/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -71,7 +71,17 @@
    * Initializes the service container.
    */
   protected function initializeContainer() {
-    // PHP identifiers always start with a letter. Hashes don't.
+    // While the default Synfomy class name only depends on the environment,
+    // for testing purposes we can't use that because there would be a
+    // collission as each test method creates a new kernel.. On the other hand,
+    // the container only depends on the enabled modules (and only on those
+    // that provide bundles) so we base the name of the container class on
+    // the hash of the enabled modules. We can't use directly the hash though
+    // because PHP identifiers always start with a letter. and hashes don't so
+    // we add a charater to the beginning. This mechanism also avoids the
+    // problem of needing to rebuild the DIC at the right time on module
+    // enable: simply on the next request the hash will change and so the
+    // container will be rebuilt.
     $class = 'c' . $this->systemList['module_enabled_hash'];
     $cache_file = $class . '.php';