Bootstrap Modal with Danger State Class?

You can but it might be dangerous.
I’ve applied panel-warning and panel-heading to the class modal-content and modal-header

  <div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
    <div class="modal-dialog">
      <div class="modal-content panel-warning">
        <div class="modal-header panel-heading">
          <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
          <h4 class="modal-title">Modal title</h4>
        </div>
        <div class="modal-body">
          ...
        </div>
        <div class="modal-footer">
          <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
          <button type="button" class="btn btn-primary">Save changes</button>
        </div>
      </div><!-- /.modal-content -->
    </div><!-- /.modal-dialog -->
  </div><!-- /.modal -->

@pimlottc has noticed the top border-radius is slightly different.
Here is the fix:

.panel-heading
{
    border-top-left-radius: inherit; 
    border-top-right-radius: inherit;
}

Check the fiddle.