Shooting a portal on the floor right next to a vertical door, then landing between the portal and the door may cause you to clip through the door. This glitch is usually done by jumping towards the door to get the right landing. This glitch can be used to save time in levels 23, 30, 35 and 38. Level 27 also has a door clip, but it seems very hard and inconsistent.

Video demonstration: Alternate strats for levels where jumping can't be used to clip through: In Level 30, you can shoot a portal at the lowest point of the left wall, enter the floor portal, and hold Right. After a short while, you'll clip right through. Level 38 has a similar strat. Make sure to move the cubes and the turret out of the way before using this strat. First, you fall between a floor portal and a ceiling portal to build up speed. Then, you shoot a portal onto the left wall, replacing the ceiling portal. If you shot the wall portal at the right height, holding Right and jumping immediately after landing should do the trick. I usually aim right above the gray rectangle on the entrance door to shoot the wall portal.