Symfony2.4 form 'This form should not contain extra fields' error

Its because when you are generating the form you are adding submit buttons but when you are validating them you are not. try:

public function cpostAction(Request $request)
{
    $entity = new Task();
    $form = $this->createForm(new TaskType(), $entity)->add('submit','submit');
    ...

The submit button is technically a field even though symfony wont map it to an entity property by default. So when you generate the form with a submit button and then submit that form the form you generate in your validation controller action needs to also have a submit button.

If you want the validator to ignore additional fields you should try passing 'allow_extra_fields' => true as an option to the FormBuilder.