Opening an unlocked but trapped door uses the key
Activating a locked and trapped door with the key in your inventory will open the door and the trap will disappear, and you see a message box that the key was used.
However, if you unlock a door with other means and leave the trap, then activate it (with the key in your inventory):
Morrowind: The trap will activate.
OpenMW: Door opens, trap disappears and you see a message box that the key was used.
One of those doors can be found in Shushishi, where Halof has the key.
This might or might not make sense the way it works in Morrowind, but that's the way it is. Maybe introduce an option?