step 1 go on Multiplayer and place an iron door
step 2 try walking/sprinting into the iron door And when you are about to hit it stop sprinting and shift and you might clip through I Don't know if you need to stop moving

The way this bug works is that 2 or 1 frames before you hit the door you crouch and then you will phase through the door but some times its not like that and i dont know why but its easy at 20 fps
and it can also be used to clip through the corner of a wall since it will push you out of the wall