Problem with breadcrumb on forum
That is because, when you visit a page that is not a node page, menu_get_object() (when called without arguments) doesn't return any value. The function could not return any value in other cases too, or not return an object, if the load function returns FALSE, as the code executed from the function is the following one.

  $router_item = menu_get_item($path);
  if (isset($router_item['load_functions'][$position]) &amp;&amp; !empty($router_item['map'][$position]) &amp;&amp; $router_item['load_functions'][$position] == $type . '_load') {
    return $router_item['map'][$position];
  }


Your code should be similar to the following one:

$node = menu_get_object();

if (arg(0) == 'forum' || (!empty($node) &amp;&amp; $node-&gt;type == 'forum')) {
  print $breadcrumb;
}