But what if you get grabbed and receive forced movement on stairs, making your Y value ambiguous until you "re-lock" it onto a stairs railing, letting you fly in the air for abit.
Then you walk across the room midair and trigger a script that collapses the 2nd floor and sends your character down by 1 floor, which, being halfway down a floor already, sends you right out of the room?
Presumably, the script triggering because the activation box is too big, or the character is still tagged "on 2nd floor" after the zombie bite which technically happened on the 2nd floor.
The constraints is usually tied to movement inputs from the player, and adding other wierd checks to "prevent characters from ever leaving that box", by snapping them back or whatever, sounds like more gamebreaking bugs waiting to happen if you ask me.
But yeah, you should totally go fix HL2, half the shit on source and unreal engines, and all the other games with your totally snazzy quick bounding box fixes!