Even in the last version of the DOOM engine, v1.9, used for Final DOOM, there is this strange limitation in which all the monsters have an invisible, infinitely tall "pillar" above and underneath them which blocks the player from, for example, running underneath a Cacodemon, or jumping down from a height above the heads of a bunch of Demons.
This was less of a problem in the original DOOM from 1993, but DOOM II and Final DOOM both have far more complex and open levels, where this frequently becomes a problem.
John Carmack fine-tuned his excellent DOOM engine for years until it stopped with v1.9. Many bugs of all kinds were fixed, and even new features were added, such as the "Nightmare" difficulty level. But this "infinite pillars" glitch/limitation was never addressed. How come? Was it actually too "expensive" CPU-wise to keep track of?
It should be noted that many later, third-party modifications of the DOOM engine have this feature (the ability to run underneath and over monsters), but then again, those engines ran on more advanced/modern computers, so maybe it only became possible to do this later?
Considering how much this changes the gameplay, and how it seems like an easy fix code-wise, was this simply a technical limitation of the kind of computers that were targeted even in 1996 (when the last DOOM game was released)?