Wikimedia Bugzilla is closed!

Here is what I found. Document is in BackCompat mode because this is at the beginning of the html:
<noscript><img src="//en.wikipedia.beta.wmflabs.org/wiki/Special:CentralAutoLogin/start?type=1x1" alt="" title="" width="1" height="1" style="border: none; position: absolute;" /></noscript>

This causes jQuery migrate to show a warning:

if ( document.compatMode === "BackCompat" ) {
	// jQuery has never supported or tested Quirks Mode
	migrateWarn( "jQuery is not compatible with Quirks Mode" );
}

migrateWarn() uses mw.track, which is not yet defined, causing JavaScript error.