And the TL;DR is that OP found com/aux/nul special names in Windows again. Which is a bit of a stretch to call it a 'mistake' if it was intentional and useful at the time (arguably still today, at least for 'nul').
I thought they found a bug in ALGOL60 code or something, which would have been funny as last week's pentest was written in an arcane language like that and likely last updated around the same time (not something we see every day; quite the opposite in terms of reinventing the wheel actually).
I agree, it's not a mistake or a bug. It's an intentional feature, followed on with intentionally including the feature in further releases.
It might not be a great feature anymore, but it's not a bug. A 40 year old bug is things like TCP didn't have timeouts in all states, so LAST_ACK sockets should stay around forever if you follow the spec, or that if the two peers get mixed up about where the seq and ack are, each ack will elicit an ack from the other side, and if the peers are near enough to each other, that can result in gigabits of useless ack traffic that never results in data exchange (thankfully this isn't a common state; I've only seen it as a result of a bug in one OSes syncookies that was fixed, and it is likely to end with a timeout)
I thought they found a bug in ALGOL60 code or something, which would have been funny as last week's pentest was written in an arcane language like that and likely last updated around the same time (not something we see every day; quite the opposite in terms of reinventing the wheel actually).