diff --git a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
index 1d49b2af18..f28726c11a 100644
--- a/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
+++ b/src/Symfony/Component/DependencyInjection/ContainerBuilder.php
@@ -326,9 +326,11 @@ class ContainerBuilder extends Container implements TaggedContainerInterface
         try {
             if (isset($this->classReflectors[$class])) {
                 $classReflector = $this->classReflectors[$class];
-            } else {
+            } elseif ($this->trackResources) {
                 $resource = new ClassExistenceResource($class, false);
                 $classReflector = $resource->isFresh(0) ? false : new \ReflectionClass($class);
+            } else {
+                $classReflector = new \ReflectionClass($class);
             }
         } catch (\ReflectionException $e) {
             if ($throw) {
diff --git a/src/Symfony/Component/Finder/Finder.php b/src/Symfony/Component/Finder/Finder.php
index e7157b1b0a..7cb0853ae7 100644
--- a/src/Symfony/Component/Finder/Finder.php
+++ b/src/Symfony/Component/Finder/Finder.php
@@ -297,6 +297,10 @@ class Finder implements \IteratorAggregate, \Countable
     /**
      * Excludes directories.
      *
+     * Directories passed as argument must be relative to the ones defined with the `in()` method. For example:
+     *
+     *     $finder->in(__DIR__)->exclude('ruby');
+     *
      * @param string|array $dirs A directory path or an array of directories
      *
      * @return $this