diff --git a/composer.json b/composer.json
index 98314db..6896603 100644
--- a/composer.json
+++ b/composer.json
@@ -24,7 +24,8 @@
         "symfony/css-selector": "^5.2",
         "symfony/console": "^5.2",
         "twig/twig": "^2.14 || ^3.3",
-        "symfony/http-client": "^5.2"
+        "symfony/http-client": "^5.2",
+        "symfony/yaml": "^5.2"
     },
     "require-dev": {
         "gajus/dindent": "^2.0",
diff --git a/composer.lock b/composer.lock
index 5e1a162..1a985fc 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "2739ab0cffd3ad3caba1283d0b2e09e2",
+    "content-hash": "c36d501ec8535171300bfa3773f593bd",
     "packages": [
         {
             "name": "doctrine/event-manager",
@@ -506,6 +506,73 @@
             ],
             "time": "2021-01-27T10:01:46+00:00"
         },
+        {
+            "name": "symfony/deprecation-contracts",
+            "version": "v2.2.0",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/deprecation-contracts.git",
+                "reference": "5fa56b4074d1ae755beb55617ddafe6f5d78f665"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/deprecation-contracts/zipball/5fa56b4074d1ae755beb55617ddafe6f5d78f665",
+                "reference": "5fa56b4074d1ae755beb55617ddafe6f5d78f665",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=7.1"
+            },
+            "type": "library",
+            "extra": {
+                "branch-alias": {
+                    "dev-master": "2.2-dev"
+                },
+                "thanks": {
+                    "name": "symfony/contracts",
+                    "url": "https://github.com/symfony/contracts"
+                }
+            },
+            "autoload": {
+                "files": [
+                    "function.php"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Nicolas Grekas",
+                    "email": "p@tchwork.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "A generic function and convention to trigger deprecation notices",
+            "homepage": "https://symfony.com",
+            "support": {
+                "source": "https://github.com/symfony/deprecation-contracts/tree/master"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2020-09-07T11:33:47+00:00"
+        },
         {
             "name": "symfony/dom-crawler",
             "version": "v5.2.4",
@@ -1516,6 +1583,81 @@
             ],
             "time": "2021-02-16T10:20:28+00:00"
         },
+        {
+            "name": "symfony/yaml",
+            "version": "v5.2.5",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/yaml.git",
+                "reference": "298a08ddda623485208506fcee08817807a251dd"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/yaml/zipball/298a08ddda623485208506fcee08817807a251dd",
+                "reference": "298a08ddda623485208506fcee08817807a251dd",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=7.2.5",
+                "symfony/deprecation-contracts": "^2.1",
+                "symfony/polyfill-ctype": "~1.8"
+            },
+            "conflict": {
+                "symfony/console": "<4.4"
+            },
+            "require-dev": {
+                "symfony/console": "^4.4|^5.0"
+            },
+            "suggest": {
+                "symfony/console": "For validating YAML files using the lint command"
+            },
+            "bin": [
+                "Resources/bin/yaml-lint"
+            ],
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Symfony\\Component\\Yaml\\": ""
+                },
+                "exclude-from-classmap": [
+                    "/Tests/"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Fabien Potencier",
+                    "email": "fabien@symfony.com"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "Loads and dumps YAML files",
+            "homepage": "https://symfony.com",
+            "support": {
+                "source": "https://github.com/symfony/yaml/tree/v5.2.5"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2021-03-06T07:59:01+00:00"
+        },
         {
             "name": "twig/twig",
             "version": "v3.3.0",
diff --git a/src/Command/BuildDocsCommand.php b/src/Command/BuildDocsCommand.php
index 8b7b7a6..e68ee15 100644
--- a/src/Command/BuildDocsCommand.php
+++ b/src/Command/BuildDocsCommand.php
@@ -153,8 +153,9 @@ protected function execute(InputInterface $input, OutputInterface $output): int
             $this->io->warning($message);
         }
 
+        $errorCount = \count($buildErrors);
         if ($logPath = $input->getOption('save-errors')) {
-            if (\count($buildErrors) > 0) {
+            if ($errorCount > 0) {
                 array_unshift($buildErrors, sprintf('Build errors from "%s"', date('Y-m-d h:i:s')));
             }
 
@@ -171,8 +172,8 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 
         $this->io->newLine(2);
 
-        if (\count($buildErrors) > 0) {
-            $this->io->success('Build completed with warnings');
+        if ($errorCount > 0) {
+            $this->io->success(sprintf('Build completed with %s errors', $errorCount));
 
             if ($input->getOption('fail-on-errors')) {
                 return 1;
diff --git a/src/DocsKernel.php b/src/DocsKernel.php
index f98b032..d1baa64 100644
--- a/src/DocsKernel.php
+++ b/src/DocsKernel.php
@@ -18,6 +18,7 @@
 use Doctrine\RST\Kernel;
 use SymfonyDocsBuilder\Listener\AssetsCopyListener;
 use SymfonyDocsBuilder\Listener\CopyImagesListener;
+use SymfonyDocsBuilder\Listener\ValidCodeNodeListener;
 
 class DocsKernel extends Kernel
 {
@@ -46,6 +47,10 @@ private function initializeListeners(EventManager $eventManager, ErrorManager $e
            PreNodeRenderEvent::PRE_NODE_RENDER,
            new CopyImagesListener($this->buildConfig, $errorManager)
        );
+        $eventManager->addEventListener(
+           PreNodeRenderEvent::PRE_NODE_RENDER,
+           new ValidCodeNodeListener($errorManager)
+       );
 
         if (!$this->buildConfig->getSubdirectoryToBuild()) {
             $eventManager->addEventListener(
diff --git a/src/Listener/ValidCodeNodeListener.php b/src/Listener/ValidCodeNodeListener.php
new file mode 100644
index 0000000..a4157b7
--- /dev/null
+++ b/src/Listener/ValidCodeNodeListener.php
@@ -0,0 +1,161 @@
+<?php
+
+declare(strict_types=1);
+
+/*
+ * This file is part of the Docs Builder package.
+ * (c) Ryan Weaver <ryan@symfonycasts.com>
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace SymfonyDocsBuilder\Listener;
+
+use Doctrine\RST\ErrorManager;
+use Doctrine\RST\Event\PreNodeRenderEvent;
+use Doctrine\RST\Nodes\CodeNode;
+use Symfony\Component\Process\Process;
+use Symfony\Component\Yaml\Exception\ParseException;
+use Symfony\Component\Yaml\Yaml;
+use Twig\Environment;
+use Twig\Error\SyntaxError;
+use Twig\Loader\ArrayLoader;
+use Twig\Source;
+
+/**
+ * Verify that all code nodes has the correct syntax.
+ *
+ * @author Tobias Nyholm <tobias.nyholm@gmail.com>
+ */
+class ValidCodeNodeListener
+{
+    private $errorManager;
+    private $twig;
+
+    public function __construct(ErrorManager $errorManager)
+    {
+        $this->errorManager = $errorManager;
+    }
+
+    public function preNodeRender(PreNodeRenderEvent $event)
+    {
+        $node = $event->getNode();
+        if (!$node instanceof CodeNode) {
+            return;
+        }
+
+        $language = $node->getLanguage() ?? ($node->isRaw() ? null : 'php');
+        if (in_array($language, ['php', 'php-symfony', 'php-standalone', 'php-annotations'])) {
+            $this->validatePhp($node);
+        } elseif ('yaml' === $language) {
+            $this->validateYaml($node);
+        } elseif ('xml' === $language) {
+            $this->validateXml($node);
+        } elseif ('json' === $language) {
+            $this->validateJson($node);
+        } elseif (in_array($language, ['twig', 'html+twig'])) {
+            $this->validateTwig($node);
+        }
+    }
+
+    private function validatePhp(CodeNode $node)
+    {
+        $file = sys_get_temp_dir().'/'.uniqid('doc_builder', true).'.php';
+        $contents = $node->getValue();
+        if (!preg_match('#class [a-zA-Z]+#s', $contents) && preg_match('#(public|protected|private) (\$[a-z]+|function)#s', $contents)) {
+            $contents = 'class Foobar {'.$contents.'}';
+        }
+
+        // Allow us to use "..." as a placeholder
+        $contents = str_replace('...', 'null', $contents);
+
+        file_put_contents($file, '<?php' .PHP_EOL. $contents);
+
+        $process = new Process(['php', '-l', $file]);
+        $process->run();
+        $process->wait();
+        if ($process->isSuccessful()) {
+            return;
+        }
+
+        $this->errorManager->error(sprintf(
+            'Invalid PHP syntax in "%s": %s',
+            $node->getEnvironment()->getCurrentFileName(),
+            str_replace($file, 'example', $process->getErrorOutput())
+        ));
+    }
+
+    private function validateXml(CodeNode $node)
+    {
+        try {
+            set_error_handler(static function ($errno, $errstr) {
+                throw new \RuntimeException($errstr, $errno);
+            });
+
+            try {
+                // Remove first comment only. (No multiline)
+                $xml = preg_replace('#^<!-- .* -->\n#', '', $node->getValue());
+                if ('' !== $xml) {
+                    $xmlObject = new \SimpleXMLElement($xml);
+                }
+            } finally {
+                restore_error_handler();
+            }
+        } catch (\Throwable $e) {
+            if ('SimpleXMLElement::__construct(): namespace error : Namespace prefix' === substr($e->getMessage(), 0, 67)) {
+                return;
+            }
+            $this->errorManager->error(sprintf(
+                'Invalid Xml in "%s": %s',
+                $node->getEnvironment()->getCurrentFileName(),
+                $e->getMessage()
+            ));
+        }
+    }
+
+    private function validateYaml(CodeNode $node)
+    {
+        // Allow us to use "..." as a placeholder
+        $contents = str_replace('...', 'null', $node->getValue());
+        try {
+            Yaml::parse($contents, Yaml::PARSE_CUSTOM_TAGS);
+        } catch (ParseException $e) {
+            if ('Duplicate key' === substr($e->getMessage(), 0, 13)) {
+                return;
+            }
+
+            $this->errorManager->error(sprintf(
+                'Invalid Yaml in "%s": %s',
+                $node->getEnvironment()->getCurrentFileName(),
+                $e->getMessage()
+            ));
+        }
+    }
+
+    private function validateTwig(CodeNode $node)
+    {
+        $twig = $this->twig ?? new Environment(new ArrayLoader());
+
+        try {
+            $tokens = $twig->tokenize(new Source($node->getValue(), $node->getEnvironment()->getCurrentFileName()));
+            // We cannot parse the TokenStream because we dont have all extensions loaded.
+            // $twig->parse($tokens);
+        } catch (SyntaxError $e) {
+            $this->errorManager->error(sprintf(
+                'Invalid Twig syntax: %s',
+                $e->getMessage()
+            ));
+        }
+    }
+
+    private function validateJson(CodeNode $node)
+    {
+        $data = json_decode($node->getValue(), true);
+        if (null === $data) {
+            $this->errorManager->error(sprintf(
+                'Invalid Json in "%s"',
+                $node->getEnvironment()->getCurrentFileName()
+            ));
+        }
+    }
+}