code: public function errorAt($timing, $method, $error = 'A mock error', $args = false, $severity = E_USER_ERROR)
      {
          $this->dieOnNoMethod($method, 'error at');
          $this->actions->registerAt($timing, $method, $args, new SimpleErrorThrower($error, $severity));
      }
docs: Sets up a trigger to throw an error upon a specific method call.
  
  @param int    $timing   When to throw the exception. A
  value of 0 throws immediately.
  A value of 1 actually allows one call
  to this method before throwing. 2
  will allow two calls before throwing
  and so on.
  @param string $method   Method name to throw on.
  @param object $error    Error message to trigger.
  @param array  $args     Optional argument list filter.
  If given then the exception
  will only be thrown if the
  method call matches the arguments.
  @param int    $severity The PHP severity level. Defaults to E_USER_ERROR.