Some Doom II-specific projectiles triggering linedefs

Several linedef types in Doom and Doom II can be triggered by actors that are not players or monsters, such as barrels. These linedefs are not intended to be activated by projectiles; however, most of the monsters introduced in Doom II are able to trigger such linedefs with their projectiles.

Explanation
The following linedef types can be triggered by non-players:

While projectiles are not supposed to trigger these actions, only the following projectiles are checked for: case MT_ROCKET:      // Rockets case MT_PLASMA:      // Plasma gun shots case MT_BFG:         // BFG9000 ball case MT_TROOPSHOT:   // Imp fireballs case MT_HEADSHOT:    // Cacodemon projectiles case MT_BRUISERSHOT: // Baron of Hell fireballs

When Doom II was released, the new monsters introduced with it did not have their projectiles added to the checklist. As a consequence, mancubus fireballs, revenant missiles, arachnotron shots and the Icon of Sin's spawn cubes are able to trigger the lift and door actions above. (The Hell knight is not affected, since it uses the same projectile as the baron.)

Usage
from 1000 Lines 2 - Community Project exploits this bug in the form of two "laser scanners" placed at specific points in the level. The player is required to lead a revenant's homing missile through the scanners to trigger them.