I was looking through some old questions trying to find bad answers to fix for editing and flagging and I came upon Stack Overflow question Closing a modal box with an iframe inside the iframe.
It completely breaks the Stack Overflow layout. I can't even see the entire question to flag to mods attention. When I checked the same question in different browsers I got a message:
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Can a moderator review it to fix the issue?
This is happening in Chrome, Firefox, and Safari. So it has to be something in the code in the question.