Can anyone identify and explain this rare Vanilla bug?

strange thing happens, which i never seen before and not see in doomwiki's bugs-n-lim list: suddenly, in a single room, middle wall texture begins scroll up like a lift continuously!

sorry, i had no demo for this, just only savegame (vanilla 1.9) for MementoMori2.wad
just type

doom2.exe -file mm2.wad -loadgame 5

heh. I think an overflow happens before.

They rise at the speed of a door.

Maybe the overflow transferred the scrolling of the opening door (either the gray one itself or the secret door) to those textures?

After loading that savegame, I didn't have to open the gray door revealing the faulty sector, to trigger the bug.

It's the ceiling of the sector (room) itself which raises, together with its pegged and unpegged wall textures. It moves like a door -- 2 units per tic, no noise. It ignores any neighbouring sector or nominal limit, though. After it overpasses its sector height limit (32767 prob'ly) -- after max. 7'48.1" -- it becomes unstable to step inside, causing Doom to quit with an error.

The faulty sector is untagged, closed, single (it's only a room, not a series of rooms). I really hope this oddity is caused by imperfect BSP (something I'm not gonna study right now -- that which involves NODES, S{ub}SECTORS and SEG{ment}S as map lump titles), but gotta yet see.

Overflow of what, entryway and myk?

I usually trigger endless movements when I block a rising so-called "platform". A "txty" platform raising from a liquid and blocked by a thing will turn back down, but forever down, sounding like a lift, not like a regular floor, and moving at slower speed. This is strange and mostly unrecommended, but's easily prevented by spacing it enough. All lowering ceilings are however designed to meet things on the floors and keep moving, slow down, pause, or raise back, depending on their purpose.

printz said:
After loading that savegame, I didn't have to open the gray door revealing the faulty sector, to trigger the bug.

Yeah, it was triggered before, probably in some obscure way, while Hitherto was playing.

It's the ceiling of the sector (room) itself which raises, together with its pegged and unpegged wall textures.

Right... now that you mention it, the ceiling is way gone above. It makes more sense too, as to why it's rising at the same rate as a normal door; I'm guessing the affected area is rising like a door.

Overflow of what, entryway and myk?

A memory overflow (entryway has managed to emulate many of these in PrBoom+ to make sure demos work as in the DOS engine). I'm not sure of what type this one is (assuming it is one), but some designs or events can cause the engine to overwrite other data, causing glitches or, after some time when the overwriting becomes severe enough (and as you noted in this case) the program's termination or crashing.

entryway said:

heh. I think an overflow happens before.

myk said:

Yeah, it was triggered before, probably in some obscure way, while Hitherto was playing.

Right. But unfortunately I missed this moment :[