Why can an ad break the Windows 11 desktop and taskbar?

Earlier today, Microsoft pushed a promotional message to early adopters of Windows 11. The promo intended to promote the upcoming operating system's integration with Microsoft Teams. Instead, it caused Explorer (the Windows desktop shell) to stop responding and left users without a working Start menu and taskbar.

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Based on the Microsoft-provided workaround, I narrowed the problem down to a registry key that contained a serialized JSON blob. The blob contained an advertisement for Microsoft Teams. The messaging and imagery in the promotion were identical to the panel you get when you press the Windows key + C on a Windows account not already set up with Teams. It's unclear if it's this exact promotion, however.

Microsoft broke every single Windows 11 computer through an ad. Windows users - you can choose a better way.