A strange architectural feature that gets noticed a lot in the Canadian province of Newfoundland is an exterior door placed rather high above the terrain, yet has no stairs leading to it. Once you've seen one of them, you can't help but notice example after example around Newfoundland. There is even an Instagram account dedicated to documenting such doors. Why would anyone install a door that would be so difficult to access and dangerous to exit? Ask any local resident, and they will tell you that's the door you show unwanted guests, hence the name "mother-in-law door." Either they won't come in, or if they are leaving, they won't come back.
But there has to be some practical reason that this, uh, custom got started. An article at 99% Invisible looked into the possibilities. One is that it's malicious compliance. After Newfoundland officially joined Canada in 1949, fire regulations demanded that buildings have two exits, but most existing homes did not. So people carved a second door into their homes. However, since the regulations did not clearly stipulate that the second exit have stairs, they didn't bother with them. Or maybe there was a tax break at some point in history for a technically "unfinished" house. Or maybe it was to deter thieves. Or it could be for an exit into high snow that would block the main door.
There is no readily available evidence for any of these theories. But there must be a reason somewhere, because you'll find not only houses but commercial buildings and even churches with mother-in-law doors!