A floor that tilts, for a real reason
The whole lower half, the red-tiled floor running up toward the bed
The floor tips up at you and the walls seem to lean, and the lazy reading is that Van Gogh simply could not do perspective. He could. The real bedroom was not rectangular; it was a trapezoid, with an obtuse angle at the front-left and an acute angle at the right, so part of the “wrong” perspective is the genuinely odd shape of the room, and part is his deliberate flattening. Both at once.