Projectiles triggering linedefs

In Doom 1.2 and lower, as well as Doom 1.666 and higher, some walk-activated linedefs can be activated by certain player and monster projectiles.

Mechanism
In early versions of Doom, things flagged MF_MISSILE were able to trigger some types of P_CrossSpecialLine linedefs triggers, such as those activating lifts. This was "fixed" in later versions of Doom with a specific exception in P_SPEC.C for the existing (at the time) projectiles:

// Things that should NOT trigger specials... switch(thing->type) {	 case MT_ROCKET: case MT_PLASMA: case MT_BFG: case MT_TROOPSHOT: case MT_HEADSHOT: case MT_BRUISERSHOT: return; break; default: break; }

When Doom II was released, three new projectile-shooting enemies were introduced (the Revenant, Arachnotron and Mancubus), and the projectiles for these monsters were not added to the exceptions list. This can be easily seen on the outdoor area of MAP09: The Pit (Doom II) where the Mancubus fireballs, when crossing any of the type 88 linedefs, lower the lift in the center. The Hell knight's fireball is functionally identical to the Baron of hell's so it is exempt.