Consistency checks: Failed Groupon email

This morning I had the usual mix of emails in my private inbox: Filtered and unfiltered spams, some emails directly for me, and a set of marketing emails. When I wanted to delete an email sent by the German Groupon site, I spotted something that didn't make sense. My first reaction was to re-read the email, then to press the button to load the "remote content" (for safety and privacy reasons this is disabled by default). Now with pictures, the content still didn't make sense:

Screenshot of Groupon email

As a database guy and also lecturer my reaction was that this will be a great talking point when the topic of consistency checks and constraints is on the agenda again. But how to make sure, I can always access it and point others to it? Here we go... ;-)