Single-use line actions can fail

In Doom and games based on it including Heretic and Strife, single-use line actions may fail to activate when the sector or sectors they target are busy, in some cases if the player or another object crosses the line from the wrong direction the first time, and in some cases if the sector or sectors they target are blocked by monsters. This will render the remainder of the map unplayable in the majority of circumstances, though it may be equally possible to exploit in other maps to avoid release of monsters or activation of traps.

Most instances of single-use actions in the game engine are coded to remove the activating linedef's special immediately upon activation. The bug is caused when these actions do not check the routine they call for a return code which indicates success or failure of the action. The Boom source port fixed this problem by checking for a true return from the action before clearing the line's special. Most other ports follow suit.