The attached file shows the effect. Start the level and shoot to wake up the chaingunner. He will wander around but will not attack you. Walk into the 3D floor pen and he will attack you. Step out again and he will stop.
Try this in an older version and the chaingunner will attack regardless of where you are standing.
For a real example, again, my Burghead mod shows the effect. The windows in the house are made with 3D floors (to allow the broken glass textures to be stacked above each other) Unfortunately, this also means that the enemies inside the house will not attack you as you make your way in (making assaulting the house far too easy) and, similarly, if you are inside the house defending against the waves of incoming enemies, the enemies won't attack you until they are in the house with you - again making it too easy. Frustratingly, of course, looking at the map again, I'd have been much better using more traditional methods. Mind you, I wouldn't have spotted the bug if I'd done that.
