1. View https://no.wikipedia.org/w/index.php?title=MediaWiki:Gadget-notifications.js&action=edit
2. Open web console

Exception in load-callback in module mediawiki.action.edit.styles:
TypeError: Cannot read property 'addButton' of undefined

[html-script]
window.RLQ.push(function() {
    mw.loader.using("mediawiki.toolbar", function() {
        mw.toolbar.addButton(false, "Bold text", "'''", "'''", "Bold text", "mw-editbutton-bold");
        ...

With this exception paused, the following evaluation can be run from the console:

mw.loader.moduleRegistry['mediawiki.toolbar']
Object { version: "RjcW87zo", dependencies: Array[0], group: null, source: "local", state: "executing"…}

Note state=executing. This should be impossible.

It appears to be triggered by the css callback from "mediawiki.action.edit.styles", which is is combined with the css callback from "mediawiki.toolbar" presumably. That in itself isn't a problem, but then it is somehow bypassing the mediawiki.toolbar module's runScript and handling the callbacks directly.