In the fortress ground floor, the player needs a key to activate the right wing lift switch. Once the player has actioned the switch to activate the lift, the player can subsequently activate the switch to deactivate the lift - and the lift toggles between active and inactive each time the player actions the switch.
The problem is, if the player has pushed the lift down from the top target, and then jumped off the lift, and then deactivated it, and then drops down the lift shaft, the player becomes trapped - see screenshot. But they will still have access to at least one save point, and, if they have previously collected the library key, they will still be able to get into the library, but the game will be unwinnable.
I guess one of the ways of fixing it would be to make the switch so that once it's activated the lift, it cannot be deactivated - but I wouldn't suggest doing this with all the lift switches in the game.