Portal Glitch Prevention
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Place Portal Bumpers around cleansers and other such objects so that the player can't portal bump past them. This glitch works on Valve's maps. How to fix: Place func_portal_bumpers around the edges of things like cleansers, and a func_noportal_volume as well.
Portal bumping on the last level of Portal. Portal bumping here allows you to really mess with the game. This is why it is so important to fix this.
Going Behind Portals
Avoid 1 unit thick walls. Players can abuse these and get behind them, although it is very difficult.
This works on one of Valve's maps. Make sure your metal walls, especially if they are detailed with some blocks sticking out, cannot be traversed.
Other design considerations
The following issues are not glitches in the sense of programming errors, but if not carefully considered by the mapper, they can be abused to circumvent parts of a puzzle and complete it in unintended ways.
Seal the bottom of Goo Pits!
A lot of people ignore this. Seal the bottom of goo pits properly. It doesn't kill instantly. Depending on the layout of your map someone may be able to get outside of or on top of it from here, and reenter later.
Watch out if your room has a portalable surface on the floor, with another on a higher section of floor, or for portalable ceiling over portalable floor. The player can abuse these to gain momentum to perform a fling. Some players have circumvented entire challenges in seconds using fling.
Jumping off switches and other things
Players can Jump onto switches and railings. Ensure they cannot jump off of these to get somewhere unintended. Example:
In the picture, you can jump from the switch to the ledge with the cube.
Portal through Portal AKA Portal Standing
Ensure the player cannot stand in one Portal, fire a portal somewhere unintended then quickly step back inside the portal they stood in to end up in an unintended area.
Portalable hallways with triggers
Ensure that any area the player could get too with a portal therefore avoiding the hallway also has a trigger.[Clarify]
Trigger_Looks and You
Always have a backup trigger in case the look doesn't get.. looked at. (where applicable)
Abusing the physics system with Prop_physics objects to climb walls. Not much you can do about this unless you have invisible blocking brushes to block areas the player shouldn't be able to get too yet.
Make sure that turret related areas don't have any unintended shields. If the player has access to a dead turret that could be used to shield, and circumvent part of the map (eg. getting a box) find a place to put a cleanser to kill the turret, or make the turret Fizzle out when dead.
Placing portals behind objects
This glitch shouldn't affect the gameplay itself, but will make your map look bad. It happens when a portal is placed too close to an object, a switch, for example, and the object will float on the portal. To avoid this, just make a brush 4 units tall, a little bit thicker and wider than the object that is affected from the glitch. Then, place the object on the new brush, group them, and place them where your switch/button or everything else should be. Now the glitch is fixed without any noportal/portalbumper surface.