For some reason, many apartment buildings require the use of a little electronic tag not only to open the outside gates, but also to operate the elevator to reach someone's apartment. This also includes trying to use the elevator to reach the ground floor, e.g., when you leave your friend's apartment and you are going home. So you can't leave the building with the elevator without your friend coming out and unlocking it for you. It's madness.
So, I clone my friends' tags (with their knowledge) and come and go as I please.