The building, used by several hundred employees, had a security systems with 4-digit codes. I've been part of group of people who liked to work late times, and the building would lock at midnight -- the box by the door would start beeping and you would need to unlock it within a minute or so, or "proper alarm" would ensue.

However, to unlock the alarm you did not need your card -- all you needed to do was to enter any valid code. Guess what was the chance that, say, 1234 was someone's valid code? Yes.

We've been all using some poor guy's code 1234, and after several years, when he left the company we just guessed some other obvious code (4321) and kept using that.

By the way, after entering the code to the box by the door, it would shortly display name of the person whom the code "belonged" to. One of our colleagues took it as a personal secret project to slowly go through all 10000 possible codes and collect the names of the people, just for the kick of it.

(By the way, I don't work for that company anymore, and more importantly, the company does not use that building anymore, so don't get any ideas! 🙃 )