YSK it isn't safe to leave your key in the door lock: it will align the pins and allow it to be opened from the outside.

This might seem obvious for the most of you, but after seeing friends doing this (thinking it would be harder to open the door from the outside), I thought I would post this anyway.

If you really like having your key in the lock, remove it halfway through to leave some of the pins in the wrong position, preventing rotation of the lock.