--- a/browser/components/customizableui/CustomizableUI.jsm
+++ b/browser/components/customizableui/CustomizableUI.jsm
@@ -175,16 +175,25 @@ var CustomizableUIInternal = {
     log.debug("Initializing");
 
     this.addListener(this);
     this._defineBuiltInWidgets();
     this.loadSavedState();
     this._introduceNewBuiltinWidgets();
     this._markObsoleteBuiltinButtonsSeen();
 
+    /**
+     * Please be advised that adding items to the panel by default could
+     * cause CART talos test regressions. This might happen when the
+     * number of items in the panel causes the area to become "scrollable"
+     * during the last phases of the transition. See bug 1230671 for an
+     * example of this. Be sure that what you're adding really needs to go
+     * into the panel by default, and if it does, consider swapping
+     * something out for it.
+     */
     let panelPlacements = [
       "edit-controls",
       "zoom-controls",
       "new-window-button",
       "privatebrowsing-button",
       "save-page-button",
       "print-button",
       "history-panelmenu",