I have a problem and I couldn't find an answer anywhere else. Perhaps it happened to you, at some moment.
I was working on my Darkplaces-based engine, and I might have screwed up something. In E1M1, the door and the elevator are opening, closing and moving up/down in one frame.
They just "flicker" open, or, in elevator's case, it "flickers" up and down. I barely manage to get through the door by keeping the "forward" key pressed.
What happened?