Is This Really A Thing?

How?
US Mint employees playing around.
There's been numerous "man made" error coins found across the US over the years where the only possibility of their very existence is that a mint employee purposely created them. Why? I don't know but I doubt they even profited from their creations unless they haphazardly risked their careers and somehow were able to successfully smuggle them out of the Mint after their shift.

A mint assisted error .

Not sure how they passed thru. metal detector, must have swallowed it

Yes it is real. There was a lot of shenanigans going on inside the late 60's early 70 at the San Francisco Mint. A lot of "assisted" errors were smuggled out of the mint. In the case of this one someone smuggled in a 1941 Canadian quarter and overstruck it with 1970 S proof quarter dies. A similar piece exists as a 1970 S proof quarter struck over a 1900 Barber quarter. (I don't remember if the Barber was an S or Philadelphia.)

And since that overstruck Canadian quarter was reported sold for that high price there have been a ton of people come forward claiming they have one. In almost every case they think they see the 1941 date (as if they used a bunch of 1941 Canadian quarters as planchets.) and in most cased their coins have been DENVER quarters. A few Philadelphia, no San Francisco's.

Cool link, thanks.