I got a JS error on MediaWiki.org when opening my Notices (Mattflaschen-WMF account):
18:05:50.724 TypeError: newNotifData.bundledNotifications.map is not a function mw.echo.Controller.prototype.fetchLocalNotifications/<() load.php:6 .Deferred/promise.then/</</<() load.php:47 jQuery.Callbacks/fire() load.php:45 jQuery.Callbacks/self.fireWith() load.php:46 .Deferred/promise.then/</</<() load.php:47 jQuery.Callbacks/fire() load.php:45 jQuery.Callbacks/self.fireWith() load.php:46 .Deferred/</deferred[tuple[0]]() load.php:47 mw.echo.api.PromisePrioritizer.prototype.setSuccess/<() load.php:37 jQuery.Callbacks/fire() load.php:45 jQuery.Callbacks/self.fireWith() load.php:46 .Deferred/</deferred[tuple[0]]() load.php:47 mw.Api.prototype.ajax/xhr<() load.php:112 jQuery.Callbacks/fire() load.php:45 jQuery.Callbacks/self.fireWith() load.php:46 done() load.php:135 .send/callback() load.php:141 1 load.php:6:648
(It failed to finish loading the flyout.)
Weirdly, it worked when I retried in debug mode, and then after that.