Linedefs can be activated regardless of player's Z position

From Doom Wiki

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.