Bootstrap 3 has-error to error conversion for CakePHP. We cannot change the error class, so use this and you get bootstrap's styling on form errors.

gistfile1.txt

/*
  Cake PHP's Form Helper will not allow use to use the has-error class, and wrap
  the form elements properly, so we update their CSS to use the .error class.
*/
.error .help-block,
.error .control-label,
.error .radio,
.error .checkbox,
.error .radio-inline,
.error .checkbox-inline {
  color: #a94442;
}

.error .form-control {
  border-color: #a94442;
  -webkit-box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.075);
          box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.075);
}

.error .form-control:focus {
  border-color: #843534;
  -webkit-box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.075), 0 0 6px #ce8483;
          box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.075), 0 0 6px #ce8483;
}

.error .input-group-addon {
  color: #a94442;
  background-color: #f2dede;
  border-color: #a94442;
}