What causes Notice: Indirect modification of overloaded property?
<blockquote>
  <p>Notice: Indirect modification of overloaded property ControllerProductProduct::$data has no effect in /home/user/public_html/example.com/catalog/controller/product/product.php on line 57</p>
</blockquote>

<p>Line 57: <code>$this-&gt;data['breadcrumbs'][] = array(</code></p>

<p>Removing <code>$this</code> from <code>$this-&gt;data</code> rids the error.</p>

<p>I don't fully understand why taking the element out of the controller makes the code work as it should?</p>