Validation error on back button in Multistep Form
Enable devel module. Then put a dpm to make sure that your code is being called.

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id){   if($form_id=='dataset_node_form')
{
 dpm($form, 'From my custom module');
 $form['actions']['previous']['#submit'][] = array(); 
 $form['actions']['previous']['#limit_validation_errors']=array(); 
}
}


If your message shows up then some other module is overriding your form_alter. You can implement hook_module_implements_alter in your custom module to make sure that the form_alter of your module is called last.