Linedefs can be activated regardless of player's Z position
In any game that uses the Doom engine, linedefs can be activated regardless of the player's Z position. This has the side effect of allowing the player to flip a switch despite being at too high or low of a height to reach it, or even if it is hidden in the floor. In certain maps, this can provide a shortcut of some degree depending on what the switch does.
An extreme example of this occurs in Hacx MAP16. The intended goal is to find and trigger four switches throughout the level, in order to reveal the exit switch, which is hidden in the floor in the center of the map, right behind where the player starts. Because of this bug, however, the player can simply turn around, and hit the switch, ending the level as soon as it begins.
Instances of the bug in the original IWADs tend to be less dramatic, but include the following.
- In Doom E2M2, there is initially a canal of slime between the player and the exit door; a nearby switch will raise a bridge over the canal, but it is behind a red door. By jumping into the slime, however, the player can click the switch through a window, thus completing the level without the red key or the blue key.
- In Doom II MAP01, it is possible to activate the switch in the northernmost room leading to the green armor secret, even if the mainframes have already been lowered into the floor.
- In Doom II MAP15, the switch in the red key tower can be clicked through a window from ground level, enabling the player to bypass the red key and go directly to the BFG area.
- In the same map, the switch on the "island" with a radiation shielding suit also can be activated from the ground level, although this is much harder.
- Almost the same situation in Doom II MAP11. The skull switch after the red key door can be pushed from the slime pool, thus allowing to skip both keys.
- In TNT: Evilution MAP31, it is possible to exit the level without keys by performing a strafe jump onto the right "throne", activating the switch, which is hidden by a lift that requires the red key to lower, and proceeding to the exit (a video showing this trick is available on YouTube).
In Hexen, due to the game's extensive programmability, this effect can be disabled manually. By using scripts, the level designer can define when a switch or any linedef can be activated. This allows one to prevent the triggering in cases where it would be unrealistic.
An example of this can be found in Darkmere, where the player needs to lower a bridge to get inside the eastern fortress. This is done by pressing an adjacent toggle that originally is hidden in the floor and needs to be lifted up by another switch. Although its function has been restricted only partially (you can hear the toggle being activated if you press the air at it), the player still has to raise it before it actually opens the bridge. So unlike in cases mentioned above, a chance for a shortcut could have been removed in this instance.
Likewise, Doom 64 includes a new linedef bit flag which enables height checking against switches.