out-of-sight-out-of-mind

The principle is essentially restricting changes to the level to things player doesn't see.
Mostly applies to dark fantasy/horror games.
The trigger for changes is just being in an "Area of Chaos"(room or area, typically a dungeon/building with confined space).
1.Architecture, content, monster, NPCs, are free to change only when player can't reach them by sight or sound. That usually means the player is at least a solid distance away:
changing things still in short term memory(about 6-12s) is more jarring and should be used sparingly.
2.Things that change don't appear out of place. They make players question their memory.
Subtle changes are more common than major restructuring.
3.Players are not notified of changes: no special sounds no glowing no movement.
4.Game can slowly "mutate" areas towards any state, provided players view all stages of mutation
5.A quest/task can also change if player doesn't see it. Mind games with memory are effective when player short-term memory is occupied by something else.
6.Player equipment can change if not viewed/used for a long time.
7.Player appearance itself is subject to change next time he looks at the mirror.
8.Multiple players "stabilize" the chaos, preventing alterations/mutations as long as they watch over the area. This restricts changes specifically when no one is looking:"a watched pot never boils".