Porkalating enemies while a death wyvern is active breaks map spots

In vanilla Hexen, transforming an enemy with a Porkalator while a death wyvern is active can corrupt the TID list by erroneously inserting things that have a TID of 0 (which should mean no TID) into spaces freed by the wyvern's activation. The wyvern must have awakened for this to happen, simple presence on the map is not enough. When the bug occurs, the most immediately apparent effect is that the wyvern will fly into a wall and become stuck, but other subtle and potentially game-breaking script errors may also result.

Saving and reloading the game after the bug has occurred will fix some of the errors; any wyverns stuck against a wall will not be able to correct their course, but any scripted effects that have not already been destroyed due to one-time triggering will become functional again.

Effect on official maps

Hypostyle is minimally affected. If one of the brown chaos serpents in the wyvern cave is Porkalated, the wyvern will fly into a wall and become trivial to defeat, but the level can still be completed normally.

Brackenwood is subject to more severe consequences if the bug is triggered. While the bug can be exploited to eliminate the wyvern threat, if the map is not fixed by saving and reloading before hitting the final switch, then the glitter bridge offering access to the emerald planet cannot appear, softlocking the game.

In ports

While the original EXEs may continue operating with anomalous behavior, ports that do not fix the bug will usually crash by segmentation fault once it has been triggered. ZDoom, Vavoom, and their derivatives (GZDoom, k8vavoom, etc.) have fixed behavior. Most other ports currently do not.

Adding checks to make sure that entities with a TID of 0 won't get added to the TID list when a monster morphs or unmorphs appears to be sufficient to remove the bug.

History

The bug was noticed by Robert Eckhardt (Ichor) in 1997 and commented upon in the text file to Death Awaits (Doomworld/idgames).