AFAICS, MediaWiki 1.26 ResourceLoader introduces a bunch of regressions, where JavaScript and CSS randomly fails loading. See for instance https://phabricator.wikimedia.org/T112047#1623903
I bet the regressions won't be fixed on time for the release, but we should at least provide sysadmins (and extension developers) with some instructions on how to debug the issues and hopefully work around them.
Nemo