diff --git a/core/modules/media/media.info.yml b/core/modules/media/media.info.yml
index 8926804..e69f14a 100644
--- a/core/modules/media/media.info.yml
+++ b/core/modules/media/media.info.yml
@@ -1,5 +1,5 @@
 name: Media
-description: 'Allows for media to be created.'
+description: 'Create reusable media.'
 type: module
 package: Core
 version: VERSION
diff --git a/core/modules/media/media.install b/core/modules/media/media.install
index e101e01..b16b348 100644
--- a/core/modules/media/media.install
+++ b/core/modules/media/media.install
@@ -15,7 +15,7 @@ function media_install() {
 
   $files = file_scan_directory($source, '/.*\.(svg|png|jpg|jpeg|gif)$/');
   foreach ($files as $file) {
-    file_unmanaged_copy($file->uri, $destination, FILE_EXISTS_REPLACE);
+    file_unmanaged_copy($file->uri, $destination, FILE_EXISTS_ERROR);
   }
 }
 
diff --git a/core/modules/media/src/Entity/Media.php b/core/modules/media/src/Entity/Media.php
index eb7e835..5b3df3b 100644
--- a/core/modules/media/src/Entity/Media.php
+++ b/core/modules/media/src/Entity/Media.php
@@ -189,7 +189,13 @@ protected function updateThumbnail($from_queue = FALSE) {
   }
 
   /**
-   * {@inheritdoc}
+   * Update the queued thumbnail for the media item.
+   *
+   * @return \Drupal\media\MediaInterface
+   *   The updated media item.
+   *
+   * @internal
+   * @todo If the need arises in contrib, consider making this a public API, by adding an interface that extends MediaInterface.
    */
   public function updateQueuedThumbnail() {
     $this->updateThumbnail(TRUE);
diff --git a/core/modules/media/src/MediaInterface.php b/core/modules/media/src/MediaInterface.php
index f9f787c..7064441 100644
--- a/core/modules/media/src/MediaInterface.php
+++ b/core/modules/media/src/MediaInterface.php
@@ -46,12 +46,4 @@ public function setCreatedTime($timestamp);
    */
   public function getSource();
 
-  /**
-   * Update the queued thumbnail for the media item.
-   *
-   * @return \Drupal\media\MediaInterface
-   *   The updated media item.
-   */
-  public function updateQueuedThumbnail();
-
 }
diff --git a/core/modules/media/src/MediaSourceInterface.php b/core/modules/media/src/MediaSourceInterface.php
index 70278a0..723e0b9 100644
--- a/core/modules/media/src/MediaSourceInterface.php
+++ b/core/modules/media/src/MediaSourceInterface.php
@@ -121,7 +121,7 @@ public function getMetadata(MediaInterface $media, $attribute_name);
    * @param \Drupal\media\MediaTypeInterface $type
    *   A media type.
    *
-   * @return \Drupal\field\FieldConfigInterface|null
+   * @return \Drupal\Core\Field\FieldDefinitionInterface|null
    *   The source field definition, or NULL if it doesn't exist or has not been
    *   configured yet.
    */
diff --git a/core/modules/media/tests/src/Kernel/MediaKernelTestBase.php b/core/modules/media/tests/src/Kernel/MediaKernelTestBase.php
index ffc4224..eb644c0 100644
--- a/core/modules/media/tests/src/Kernel/MediaKernelTestBase.php
+++ b/core/modules/media/tests/src/Kernel/MediaKernelTestBase.php
@@ -77,7 +77,11 @@ protected function setUp() {
     ]);
     $this->testConstraintsMediaType->save();
     $source_field = $this->testConstraintsMediaType->getSource()->createSourceField($this->testConstraintsMediaType);
+    // The media type form creates a source field if it does not exist yet. The
+    // same must be done in a kernel test, since it does not use that form.
+    // @see \Drupal\media\MediaTypeForm::save()
     $source_field->getFieldStorageDefinition()->save();
+    // The source field storage has been created, now the field can be saved.
     $source_field->save();
     $this->testConstraintsMediaType->set('source_configuration', [
       'source_field' => $source_field->getName(),