Picture a room with an interactive sector portal. The top layer contains a 3D floor with enemies on it, the bottom contains nothing but a steep slope under said 3D floor. You'd expect nothing spectacular to come out of this, but the slope (it has to be 45 degrees or more) influences the top layer in a bizarre way: monsters can not get past that spot where the slope would be, even though they're walking on a flat 3D floor (Flyers are an exception of course).
I recorded a video: https://www.youtube.com/watch?v=R_lAo1kxH5A
And here's the WAD itself: https://drive.google.com/file/d/1DwYiVv ... sp=sharing