Apparently the translation file on DemoBundle stopped working after upgrading to Symfony 2.5.x
Make a clean installation using Demo with content
Click on "Contact Us"
Add a name with over 255 characters
Fill the rest of the form
Click on "Send form" button
You'll notice the error, next to the First name "feedback.max_size.255" when it should be "This field cannot be longer than 255 characters"