Running 3.0.5 and when I make simple changes (add a field for example) and run /dev/build?flush=1, I get a blank screen. I'm in dev mode, no errors are kicked out. Adding a field shouldn't be so difficult! I keep experiencing this with a few of my sites and they are simple ones. See below... not sure how this can break anything! It's a fresh install.

<?php
class Page extends SiteTree {

   public static $db = array(
   'Right' => 'HtmlText'
   );

   public static $has_one = array(
   );
   
   function getCMSFields() {
$fields = parent::getCMSFields();
$fields->addFieldToTab('Root.Content.Right', new HTMLEditorField('Right'));

return $fields;
}

}
class Page_Controller extends ContentController {

   /**
    * An array of actions that can be accessed via a request. Each array element should be an action name, and the
    * permissions or conditions required to allow the user to access it.
    *
    * <code>
    * array (
    * 'action', // anyone can access this action
    * 'action' => true, // same as above
    * 'action' => 'ADMIN', // you must have ADMIN permissions to access this action
    * 'action' => '->checkAction' // you can only access this action if $this->checkAction() returns true
    * );
    * </code>
    *
    * @var array
    */
   public static $allowed_actions = array (
   );

}

Also Matt it is good practice to enable Director::set_environment_type('dev'); in your "mysite/_config.php" so it can give you an error. Usually a good indicator of what is wrong in your code.

Regards