To replicate, go to any page in your browser console:

If jQuery UI is loaded:

• typeof $.widget -> function
• typeof $.widget.extend -> function

If it isn't:

• typeof $.widget -> undefined
• typeof $.widget.extend -> TypeError

Then click to open your Echo notifications. On wikis where I've never received notifications, nothing changes. On wikis where I have any number of notifications, past or present, I'll now see:

• typeof $.widget -> object
• typeof $.widget.extend -> undefined

This can break things that rely on jQuery UI if they're used after someone checks their notifications.