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.
yeah I've found some other usage cases for this, I'm Kruin#7777 on disc if you want to discuss, I'm going to start grinding any% no level skips after I finish with minecraft
I found a consistent way to get the level 27 door clip
Everything is normal except you place your crosshair in the center of the vent at the top right door. This seems to work every time for me
New clip on 21: shoot one portal right next to the door, shoot the other on the floor, walk through floor portal and hold Right
I'm picking the game up again, the creation of a category with all glitches besides the level skip glitch would be good, I have some tech for level 36 I think. I'll try to upload some of the stuff I've found so we can all compare. would be cool to see some optimization in this game
