[OJFord]

Never heard of it London, mine certainly has at least one: I had a guest of a neighbour on a (very) different floor try for some time to open my door - with a key - in the middle of the night one time...

[Symbiote]

My friend had someone sleep on his floor after mistaking his door for the apartment above.

Before that, I was pretty lax with locking the door at night.