Not just [pressing lock, but those who are using the night-latch system at their home or office can narrate a few incidents when they had to suffer as they forgot to take the keys with them, when they went out.
There was the report of a very sad event in the media in Kerala some time ago. An young woman staying in a flat, with her small child, was standing outside the front door,bu the door suddenly closed behind her. She did not take the keys. The child was inside. It started crying The mother got panicked and wanted to get in to be with the lonely and scared infant somehow. Though the society watchman pacified her and went down to bring some remedial measure, she was becoming more panicky and impatient. Meanwhile she seems to have tried to enter her balcony side by climbing on the parapet of the flat and extending to the balcony. She lost the balance and grip and fell down fatally injured. It makes me shudder to even recall about the report of the event.
I can narrate a hilarious (but real) incident also in this regard.
About forty years ago, one of my colleague who was not familiar with the night latch system had faced a situation when he went out without taking the keys. He panicked and called the neighbours for help. He was worried that his newly married wife, who is new to the place has got stuck inside. He was really panicky as to what will happen now. He was telling them to break open the door.
Then telling him to keep quite, a middle aged woman pressed the door bell. The door opened and the wife came out wondering why all the hullaballa. In night latch person inside can open the lock, but from outside, it needs key.