From a conversation with Simon on Mumble. He suggested that Zombies might un-push Buttons, which I think is a great idea.
I liked the idea so much that was about to go ahead and implement the change immediately, but he then pointed out that it would mean they shouldn't then interact with Buttons in their regular state, and also asked what should happen if the Exit is already unlocked.
The first of these points does present an interesting issue - Zombies can now exit, so it's in their interest to push buttons to unlock the Exit (it's one of the reasons I implemented Zombie interaction with buttons in the first place). However, their presence in the level is generally negative - they eat pickups, count as -1 towards the save requirement, ignore skill assignments, and zombify other lems. So, it makes sense that their interaction with buttons should also be negative. More discussion is needed on this.
Meanwhile, if the Exit is already unlocked, it should of course be re-locked if a Button is un-pressed. We'd have to display the unlocking animation in reverse (easy enough, hopefully), but it might not be as simple as even that.
Discussion welcome!
