Back in the day, no link on a google domain that pointed to a google domain was a 404 link. That's right, zero 404s, site wide.
So the 404 page was pretty much only seen by IT folk--people who would type in their own URLs instead of clicking links...
And those people, the people who type a lot... had probably seen the old meme in which the trailing ! in a string of !!! is replaced by a 1, as would happen if you were typing quickly and let go of shift a little early...
WELCOME TO THE FUTURE OF THE OPEN INTERNET /s