diff --git a/src/Error/Warning.php b/src/Error/Warning.php
new file mode 100644
index 0000000..0d2e299
--- /dev/null
+++ b/src/Error/Warning.php
@@ -0,0 +1,24 @@
+<?php
+namespace GraphQL\Error;
+
+final class Warning
+{
+    static $supressWarnings = false;
+
+    static $warned = [];
+
+    static function supress($set = true)
+    {
+        self::$supressWarnings = $set;
+    }
+
+    static function warnOnce($errorMessage, $errorId = null)
+    {
+        $errorId = $errorId ?: $errorMessage;
+
+        if (!self::$supressWarnings && !isset(self::$warned[$errorId])) {
+            self::$warned[$errorId] = true;
+            trigger_error($errorMessage, E_USER_WARNING);
+        }
+    }
+}
diff --git a/src/Utils.php b/src/Utils.php
index 45b10a0..d1a2c68 100644
--- a/src/Utils.php
+++ b/src/Utils.php
@@ -2,6 +2,7 @@
 namespace GraphQL;
 
 use GraphQL\Error\InvariantViolation;
+use GraphQL\Error\Warning;
 use GraphQL\Type\Definition\Type;
 use GraphQL\Type\Definition\WrappingType;
 use GraphQL\Utils\SchemaUtils;
@@ -351,9 +352,10 @@ class Utils
         }
 
         if (!$isIntrospection && isset($name[1]) && $name[0] === '_' && $name[1] === '_') {
-            throw new InvariantViolation(
+            Warning::warnOnce(
                 'Name "'.$name.'" must not begin with "__", which is reserved by ' .
-                'GraphQL introspection.'
+                'GraphQL introspection.',
+                'warnAboutDunder'
             );
         }
 
diff --git a/tests/Type/ValidationTest.php b/tests/Type/ValidationTest.php
index 79fbead..5ab9eb3 100644
--- a/tests/Type/ValidationTest.php
+++ b/tests/Type/ValidationTest.php
@@ -34,7 +34,7 @@ class ValidationTest extends \PHPUnit_Framework_TestCase
 
     public function testRejectsAnObjectTypeWithReservedName()
     {
-        $this->assertEachCallableThrows([
+        $this->assertWarnsOnce([
             function() {
                 return new ObjectType([
                     'name' => '__ReservedName',
@@ -53,6 +53,7 @@ class ValidationTest extends \PHPUnit_Framework_TestCase
             function() {
                 return new UnionType([
                     'name' => '__ReservedName',
+                    'types' => [new ObjectType(['name' => 'Test'])]
                 ]);
             },
             function() {
@@ -268,6 +269,26 @@ class ValidationTest extends \PHPUnit_Framework_TestCase
         }
     }
 
+    private function assertWarnsOnce($closures, $expectedError)
+    {
+        $warned = false;
+
+        foreach ($closures as $index => $factory) {
+            if (!$warned) {
+                try {
+                    $factory();
+                    $this->fail('Expected exception not thrown for entry ' . $index);
+                } catch (\PHPUnit_Framework_Error_Warning $e) {
+                    $warned = true;
+                    $this->assertEquals($expectedError, $e->getMessage(), 'Error in callable #' . $index);
+                }
+            } else {
+                // Should not throw
+                $factory();
+            }
+        }
+    }
+
     private function schemaWithFieldType($type)
     {
         return new Schema([