Ext.panel.Header.setIconCls does not respect the condition that the header might no-yet be rendered. The Header's rendering may - however - be independent of the 'main' panels rendering.

E.g.: Loading a background tab in a tabpanel.

Instead of correctly falling back in all custom objects (which would include custom iconchange event handling et al), I suggest handling the not-yet-rendered case directly in Ext.panel.Header. The attached patch works for me so far

Code:
diff --git a/src/panel/Header.js b/src/panel/Header.js
index 92520f9..d5cee2f 100644
--- a/src/panel/Header.js
+++ b/src/panel/Header.js
@@ -290,9 +290,6 @@ Ext.define('Ext.panel.Header', {
      */
     setIconCls: function(cls) {
         this.iconCls = cls;
-        if ( !this.rendered ) {
-            return;
-        }
         if (!this.iconCmp) {
             this.initIconCmp();
             this.insert(0, this.iconCmp);