Go to the part in the very first episode where Toad and Kurt are fighting. When they go down the corridor, they disappear on the right side of the screen, indicating that they went down the corridor visible on the left side of the stairs. Now, fast foward through The Prof.'s bit. Now watch the scene where Kurt and Toad are coming out of the doorway. They appear around a corner on the left side of the screen, indicating that they went behind the stairs and came out on the other side. But when Kurt is climbing on the bottom of the upper stairs in the next scene, you can see that they are still on the left side of the stairs!