.../EntityResource/EntityResourceTestBase.php      | 36 +++++++++++++++++++---
 .../rest/tests/src/Functional/ResourceTestBase.php |  4 +--
 2 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
index eea476f..15690d6 100644
--- a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceTestBase.php
@@ -414,15 +414,14 @@ public function testGet() {
     // DX: 406 when requesting unsupported format.
     $response = $this->request('GET', $url, $request_options);
     $this->assert406Response($response);
-    // @todo this works fine locally, but on testbot it comes back with
-    // 'text/plain; charset=UTF-8'. WTF.
-//      $this->assertSame(['text/html; charset=UTF-8'], $response->getHeader('Content-Type'));
+    $this->assertNotSame([static::$expectedErrorMimeType], $response->getHeader('Content-Type'));
 
 
     $request_options[RequestOptions::HEADERS]['Accept'] = static::$mimeType;
 
 
     // DX: 406 when requesting unsupported format but specifying Accept header.
+    // @todo Update in https://www.drupal.org/node/2825347.
     $response = $this->request('GET', $url, $request_options);
     $this->assert406Response($response);
     $this->assertSame([static::$expectedErrorMimeType], $response->getHeader('Content-Type'));
@@ -460,7 +459,7 @@ public function testPost() {
     $parseable_invalid_request_body   = $this->serializer->encode($this->makeNormalizationInvalid($this->getNormalizedPostEntity()), static::$format);
     // @todo Change to ['uuid' => UUID] in https://www.drupal.org/node/2820743.
     $parseable_invalid_request_body_2 = $this->serializer->encode($this->getNormalizedPostEntity() + ['uuid' => [['value' => $this->randomMachineName(129)]]], static::$format);
-    $parseable_invalid_request_body_3 = $this->serializer->encode($this->getNormalizedPostEntity() + ['field_rest_test' => [['value' => 'no access value']]], static::$format);
+    $parseable_invalid_request_body_3 = $this->serializer->encode($this->getNormalizedPostEntity() + ['field_rest_test' => [['value' => $this->randomString()]]], static::$format);
 
     // The URL and Guzzle request options that will be used in this test. The
     // request options will be modified/expanded throughout this test:
@@ -599,6 +598,21 @@ public function testPost() {
     $this->assertNormalizationEdgeCases('POST', $url, $request_options);
     $this->assertAuthenticationEdgeCases('POST', $url, $request_options);
 
+
+    $request_options[RequestOptions::HEADERS]['Content-Type'] = 'text/xml';
+
+
+    // DX: 415 when request body in existing but not allowed format.
+    $response = $this->request('POST', $url, $request_options);
+    // @todo Update this in https://www.drupal.org/node/2826407. Also move it
+    // higher, before the "no request body" test. That's impossible right now,
+    // because the format validation happens too late.
+    $this->assertResourceErrorResponse(415, '', $response);
+
+
+    $request_options[RequestOptions::HEADERS]['Content-Type'] = static::$mimeType;
+
+
     // 201 for well-formed request.
     $response = $this->request('POST', $url, $request_options);
     $this->assertResourceResponse(201, FALSE, $response);
@@ -793,6 +807,20 @@ public function testPatch() {
     $this->assertAuthenticationEdgeCases('PATCH', $url, $request_options);
 
 
+    $request_options[RequestOptions::HEADERS]['Content-Type'] = 'text/xml';
+
+
+    // DX: 415 when request body in existing but not allowed format.
+    $response = $this->request('PATCH', $url, $request_options);
+    // @todo Update this in https://www.drupal.org/node/2826407. Also move it
+    // higher, before the "no request body" test. That's impossible right now,
+    // because the format validation happens too late.
+    $this->assertResourceErrorResponse(415, '', $response);
+
+
+    $request_options[RequestOptions::HEADERS]['Content-Type'] = static::$mimeType;
+
+
     // 200 for well-formed request.
     $response = $this->request('PATCH', $url, $request_options);
     $this->assertResourceResponse(200, FALSE, $response);
diff --git a/core/modules/rest/tests/src/Functional/ResourceTestBase.php b/core/modules/rest/tests/src/Functional/ResourceTestBase.php
index 6fb0fa5..4822fa1 100644
--- a/core/modules/rest/tests/src/Functional/ResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/ResourceTestBase.php
@@ -344,9 +344,7 @@ protected function assertResourceResponse($expected_status_code, $expected_body,
    *   The error response to assert.
    */
   protected function assertResourceErrorResponse($expected_status_code, $expected_message, ResponseInterface $response) {
-    // @todo Either add this to \Drupal\serialization\Encoder\JsonEncoder, or
-    //   figure out how to let tests specify encoder options, and figure out
-    //   whether they should apply to just error responses or to everything.
+    // @todo Fix this in https://www.drupal.org/node/2813755.
     $encode_options = ['json_encode_options' => JSON_HEX_TAG | JSON_HEX_APOS | JSON_HEX_AMP | JSON_HEX_QUOT];
     $expected_body = $this->serializer->encode(['message' => $expected_message], static::$format, $encode_options);
     $this->assertResourceResponse($expected_status_code, $expected_body, $response);