Funny story, possibly apocryphal since it's a friend of a friend kind of deal. Anyway, one person in a household gets up early every morning for work and wants to lock the door so that people inside are safe, but also so that they have the key. The lock wasn't the sort that automatically locks when someone leaves.

Their usual routine was to lock the door then post the key back through the letter slot in the door.

One day, this person's neighbour finds them trying and failing to reach in through the letter slot in the door in order to get the key back.

"Why are you doing that? What's wrong?"

"I've forgotten to lock the door before putting the key back through. Can you get me something like a coat hanger or something so I can reach it?"

"So, it's not locked?"

"No! And it needs to be!"

"Why can't you just open it?"

"I don't have the key!"

The neighbour had to grab the handle and open the door to resolve the confusion.