Ah, I have never tried to use ed interactively (I've worked with sed and I mostly edit in vi or its relatives but even the oldest machine I own has a video terminal so I don't need ed) but that makes sense as the source of the '?' error.

I'm aware that the story isn't about security, maybe that didn't come through in my post, it's just that the story always comes to mind when talking about how verbose to make error messages. As you illustrate, there are real trade offs, just today's trade-offs are different than in Ken's PDP programming days.