Noticed whilst testing MDL-32745:

Open a course and choose to add a new URL.
Open your JS console
Refresh the page a few times

Uncaught Error: extend failed, please check that all dependencies are included.

Backtrace points to undefined Overlay var in build/menu/menu.js line 1187:

YAHOO.lang.extend(Menu, Overlay, {

We don't see this on every request