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IDProjectCategoryView StatusDate SubmittedLast Update
0002272Zandronum[All Projects] Bugpublic2015-05-29 08:042026-05-19 19:45
ReporterFrits 
Assigned To 
PrioritynormalSeverityminorReproducibilityalways
StatusconfirmedResolutionopen 
PlatformMicrosoftOSWindowsOS VersionXP/Vista/7
Product Version2.0 
Target VersionFixed in Version 
Summary0002272: Monster movement prediction on slopes
DescriptionI've attached a demo showing the problem. The monster seems to warp down the slope and eventually warps back to the top of the slope.
Steps To ReproduceI know it's a hefty wad list, but just placing an imp on a slope for a minimal wad doesn't work. I'll tinker around a bit more to see if i can make one though.

1) playback demo
Wads used: doom2.wad aliens2014fix.pk3 aliens0.99g2.pk3 endless_v04r2.pk3 doommetalvol4.wad newtextcolours1_170.pk3
Attached Files? file icon 2015.05.29_09.52.30_doom2.aliens2014fixpk3.cld [^] (905,160 bytes) 2015-05-29 08:04
? file icon monster_slope_desync_01.wad [^] (14,770 bytes) 2026-05-19 19:42
? file icon 2026.05.19_21.37.04_doom2.monster_slope_desync_01.cld [^] (63,760 bytes) 2026-05-19 19:43

- Relationships

-  Notes
User avatar (0024752)
unknownna (updater)
2026-05-19 19:45

 I can confirm the bug, it seems to be triggered by a client having some latency when interacting with the alien on the slope. For some reason, this causes the alien to bug up for all clients on the server. It's very hard to reproduce reliably, and it only seemingly breaks during online play.

It seems to be caused by a combination of A_Jump* specials + fast tick states and ThrustThing, though it actually breaks the monster on the server as well, so it's not just a desync between the client and server.

1. zandronum -iwad doom2.wad -file monster_slope_desync_01.wad -host +sv_updatemaster 0 +sv_cheats 1
2. Connect a client to the server with an emulated ping of 100 and join the game. Enable noclip so you can bait the demon to lunge at you.
3. Try to move so that the demon lunges at you from above before you drop out of sight. With noclip, you can re-position yourself for the attack and leave once the demon has lunged at you. If successful, the demon will start to glitch at a spot and move back and forth in an erratic and jittery way.

I also recorded a lucky demo of reproducing it quickly it with the latest beta build (Zandronum 3.3-260112-1855) using the example wad.

1. zandronum -iwad doom2.wad -file monster_slope_desync_01.wad -playdemo 2026.05.19_21.37.04_doom2.monster_slope_desync_01.cld

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- Issue History
Date Modified Username Field Change
2015-05-29 08:04 Frits New Issue
2015-05-29 08:04 Frits File Added: 2015.05.29_09.52.30_doom2.aliens2014fixpk3.cld
2026-05-19 19:42 unknownna File Added: monster_slope_desync_01.wad
2026-05-19 19:43 unknownna File Added: 2026.05.19_21.37.04_doom2.monster_slope_desync_01.cld
2026-05-19 19:45 unknownna Note Added: 0024752
2026-05-19 19:45 unknownna Status new => confirmed




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