On BOAC 707s we had them, but for the life of me I cannot remember exactly how they actually worked, but here goes!
From my vague memory, there was a stainless steel bar which was located at the base of the evacuation pack on the door, and when we closed a door, this bar had has to be clipped into two 'hooks' on the floor, on each side, at the base of the door. The so-called 'arming'!
The opposite had to be done before opening the doors, unclip the steel bar and then open the door. The door had to be opened by a crew member inside the aircraft, not by the ground staff outside, as done now!
Yes, it did happen - sometimes the doors were opened without unclipping that steel bar, with the result of the slide expanding inside the aircraft. It never happened on any of my trips - thank goodness!
Per Ardua ad Astra! ........ Honi Soit Qui Mal y Pense!