To go through ceilings (open boxes stacked on top of each other), you must be on a Forge map (a map that is used on Forge and custom games). Place a door horizontally on the second story box and place a machine gun turret to where it is laying down on the floor, but the part you grab onto is propped up onto the door. Make sure the part you grab onto is facing the floor. After a couple seconds, delete the door underneath the turret. The turret should float like that. Whenever you play that particular custom map on Forge or custom games, you should be able to jump up underneath that turret and grab onto it through the ceiling, then release it and pass through the ceiling. Note: This only works with machine gun turrets.