Error page of the error page made me smile. Somehow, this makes total sense to me. Consider a reverse proxy where the origin is down and displaying the 'real' error page is not possible because of that. Alternatively, imagine a CloudFront function or Lambda experiencing the same issue, or encountering so many redirects that it interrupts and simply shows the error page of the error page. Nonetheless, I agree that you shouldn't see this issue very often.
I had this for a reverse proxy I developed that did some transformations. At about two critical points if there was an error there was literally nothing we could do except 500 barf.
