Engine bug

A Doom engine bug is a limitation or an oddity present in the Doom engine. For errors in map design, even those related to a specific item below, see the article about that map.

This article uses the version 1.9 DOOM2.EXE DOS version as the base version. Older versions, the source release, Final Doom and The Ultimate Doom have some minor changes, tweaks and different bugs.

Note that playing with a source port may, while removing some or even the majority of the bugs listed below, introduce other bugs which are not listed here (although details may be present in the article about the source port in question).

Key

This table classifies anomalies in a very broad way; see the individual articles for details.

• Canonicity — A bug is verified only when a current or former id Software programmer has called it a bug in a public medium. An unverified bug may still be undisputed if it is generally accepted as an error or issue by the fan community under a consensus involving thorough playtesting and research on the corresponding phenomenon and any associated binaries or source code. A bug is disputed if it can be argued to be a harmless and possibly intentional feature.

• Cause — The general category of underlying problem:
  • Most bugs are due to algorithms which fail to account for all possible inputs, apply conditional statements in an illogical sequence, or have unforeseen consequences in particular game situations.
    • A few of these arise from simple typos in the source code.
  • Improperly constructed linedefs (with orphaned tags or incorrectly placed textures) or sectors, can also induce various strange behaviors.
  • The Doom engine includes few safeguards against overflow conditions.
  • Line-of-sight calculations, rendering algorithms, and the BSP tree are also susceptible to roundoff errors.
  • The engine imposes a number of static limits on Thing placement and map construction, which sometimes fix one problem by creating another.

• Fatal bugs are those where the engine crashes (often with the Venetian blind effect) or else exits the game in a controlled fashion, usually with an error message. Y* means that termination is possible, but not inevitable. N* means that the program keeps running, but that rendering or character behavior may be sufficiently compromised that meaningful gameplay becomes impossible.

• A bug has a workaround if it can be avoided by reasonable compromises in map design, such as when matching every linedef tag to at least one sector, but not when removing invulnerabilities from every level containing a sky texture, or also by convenient precautions and other measures taken by the program user, such as when increasing the mouse sensitivity through the configuration file directly, instead of using the menu. S means that the bug can be avoided only by making one's map smaller or less complex.

• A bug is a loophole if it can be abused to the player's advantage (especially during speedruns or deathmatches).

Bugs present in Doom II version 1.9 for DOS

Phenomenon	Canonicity	Cause	Fatal?	Workaround?	Loophole?
(0,0) respawning bug	Undisputed	Algorithm	N	N	Y
1% damage bug		Algorithm	N	N	N
Absurd texture name in error message	Undisputed	Algorithm	Y*	N	N
AASTINKY and AASHITTY	Undisputed	Algorithm	N	N	N
All-ghosts effect	Undisputed	Overflow	N*	N	Y
Anomalous angry face behavior		Algorithm	N	N	N
Arch-vile fire spawned at the wrong location	Undisputed	Typo	N	N	N
Baron attacks a monster behind him		Algorithm	N	N	N
Barrel explosions which do no damage	Undisputed	Algorithm	N	N	N
Blast damage has unlimited vertical range	Undisputed	Algorithm	N	N	Y
Blockmap overflow	Undisputed	Overflow	N	S	N
Boss level ending not triggered if every player is dead	Undisputed	Algorithm	N*	N	N
Bullet puffs do not appear in outdoor areas		Algorithm	N	N
Chaingun makes two sounds firing single bullet	Undisputed	Algorithm	N	N	N
Chainsaw negates projectile impact thrust	Undisputed	Algorithm	N	N	Y*
Colormap blink ignores concurrent powerup [1]			N	N	N
Crash when accessing Read This! [2]	Undisputed		Y	N	N
Crushed barrels leave behind a pool of blood	Undisputed	Algorithm	N	Y	N
Demo desyncing caused by bouncing lost souls	Undisputed	Algorithm	N	N	N
Demo desyncing caused by menu access	Undisputed	Algorithm	N	N	N
Demon speed bug		Algorithm	Y*		Y
Doom II monster exclusion bug	Undisputed	Algorithm	Y*	Y	N
Drawsegs overflow	Undisputed	Static limit	N	S	N
Elastic collisions with walls	Undisputed	Roundoff	N	N	Y
Fast doors make two closing sounds	Undisputed	Algorithm	N	N	N
Fast doors reopening with wrong sound	Disputed	Algorithm	N	N	N
Final Doom teleporters do not set Z coordinate	Undisputed	Algorithm	N	N	N
Firing frame position not updated when the player dies		Algorithm	N	N	N
Flawed collision detection	Undisputed	Algorithm	N	N
Floors and ceilings moving silently	Undisputed		N	N	Y
Frag count rollover		Overflow	N	N	N
Gamma correction resets palette	Undisputed	Algorithm	N	N	Y
Ghost monsters	Undisputed	Algorithm	N	N	N
GUS instrument mappings bug	Undisputed	Algorithm	N	Y	N
Hall of Mirrors effect (limit)	Undisputed	Algorithm	N	S	N
Hall of Mirrors effect (missing texture)	Undisputed	Linedef	N	Y	N
Health percentage hysteresis in sector type 11		Algorithm	N	N	N
Hitscan attacks hit invisible barriers in large open areas	Undisputed	Overflow	N
IDCHOPPERS cheat confers 1 tic of invulnerability		Typo	N	N
IDCLEV idiosyncrasies			N	N	N
IDMUS requests invalid music		Algorithm	Y	N	N
Inaccurate trigonometry table	Undisputed	Algorithm	N	N	N
Invulnerability colormap bug	Disputed	Algorithm	N	N	N
joyb_speed magic numbers		Overflow	N	Y	Y
Key pickup resets palette	Undisputed	Algorithm	N	N	N
Kill percentage flag used for -nomonsters and respawning	Disputed	Algorithm	N*	N	Y
Linedef type 37			Y	Y	N
Linedefs can be activated regardless of player's Z position	Undisputed	Algorithm	N	N	Y
Long wall error	Undisputed	Roundoff	N	S	N
Lopsided final boss explosions	Undisputed	Typo	N	N	N
Lost soul charging backwards	Disputed	Algorithm	N	N	Y
Lost soul clipping problems	Undisputed	Algorithm	N	N	N
Lost soul colliding with items	Undisputed	Algorithm	N	N	Y
Lost soul limit	Undisputed	Static limit	N	N	Y
Lost soul target amnesia	Disputed	Algorithm	N	N	N
Lump size limit		Static limit		S	N
Mancubus fireball clipping	Undisputed	Algorithm	N	N	N
Manual doors on 1S lines cause an illegal operation	Undisputed	Linedef	Y*	Y	N
Map size limit (blocks)	Undisputed	Static limit	Y	S	N
Map size limit (integer overflow)		Static limit	N	N	N
Medusa effect	Undisputed	Algorithm	N*	Y	N
Menu error with high mouse sensitivity	Undisputed	Overflow	Y	Y	N
Monsters attacking across tall ledges	Disputed	Algorithm	N	N	N
Monsters fleeing	Disputed	Algorithm	N	N
Monsters may target players without ever seeing or hearing them	Disputed	Algorithm	N	N	N
Monsters open locked doors	Undisputed	Roundoff	N	N	Y
Monsters stuck in doortracks, walls or hanging off lifts	Undisputed	Algorithm	N	N	Y
Moving platforms limit	Undisputed	Linedef	Y	S	N
Moving platform heights not preserved in saved games	Undisputed	Algorithm	N	N	N
Noclip bug	Undisputed	Algorithm	N	N	Y
Non-Newtonian motion of corpses	Disputed	Algorithm	N	N	Y
Numeric overflow crash in tall areas	Undisputed	Overflow	Y*	S	N
Ouch face	Verified [3]	Typo	N	N	N
Par times hidden in Final Doom after idclev or reload			N	N	N
Partial invisibility's fuzz effect stays still		Algorithm	N	N	N
Partial invisibility stippling algorithm	Disputed	Algorithm	N	N	N
Pause bug in menus with scroll bars	Undisputed	Algorithm	N	N	N
Phantom fireball explosions	Disputed	Algorithm	N		N
Picked up a medikit that you REALLY need!	Undisputed	Typo	N	N	N
Player corpses walking	Undisputed	Overflow	N*	Y	N
Player face grins after restoring savefile	Disputed	Algorithm	N	N	N
Player starts missing	Undisputed		Y	Y	N
Player stuck on ledge after restoring savefile	Undisputed	Algorithm	N*	N	N
Projectiles disappear when hitting walls	Undisputed	Algorithm	N	Y	N
Projectiles explode on impact with "sky"		Algorithm	N	N	N
PWAD size limit	Undisputed	Algorithm	Y	Y	N
Quantum lost soul	Disputed	Algorithm	N	N	N
Raising sectors change to orphaned sector type	Disputed	Sector	N	Y	N
Random sound pitch removed			N	N	N
Ranged attacks blocked by decorations	Disputed	Algorithm	N	N	Y
Recurrent blast damage	Undisputed	Algorithm	N	S	Y
Revenant tracers desync internal demos	Undisputed	Algorithm	N*	N	N
Rocket passes through the player who fired it	Disputed	Algorithm	N	N	Y
Savegame buffer overflow	Undisputed	Static limit	Y	S	N
Screen melt effect during Doom II cast sequence		Algorithm	N	N	N
Secret exit idiosyncrasies		Algorithm	N	Y	N
Secret sectors appear or disappear		Algorithm	N	S	N
Secret sectors have no geometry		Algorithm	N	Y	N
Sector type 17 is disabled after loading a saved game	Undisputed	Algorithm	N	N	N
Shotgun sprite wraps vertically in Final Doom		Algorithm	N	Y	N
Silent BFG trick	Undisputed	Static limit	N	N	Y
Single-use line actions can fail	Undisputed	Algorithm	N*	N	Y
Sky never changes in Doom II	Undisputed	Algorithm	N	N	N
Sleeping shotgun guy in MAP02 (Doom II)	Undisputed	Typo	N	N
Slime trails	Undisputed	Roundoff	N	N	N
Some Doom II-specific projectiles triggering linedefs	Verified	Algorithm	N	N	Y
Some game options not preserved in saved games	Undisputed	Algorithm	N	N	N
Sound cutoffs	Disputed	Static limit	N	N	N
Sound effects behave differently on level 8	Disputed	Algorithm	N	N	N
Sounds changing pitch on slow computers			N	N
Spawn cubes miss east and west targets	Undisputed	Algorithm	N	Y	N
Spawn fog in wrong location	Undisputed	Typo	N	N	N
Spawn spots not preserved in saved games	Undisputed	Algorithm	Y*	N	Y
Spechits overflow	Undisputed	Static limit	Y	S	N
Spider paralysis	Undisputed	Algorithm	N	N	Y
Sprites flickering across ledges or lifts	Undisputed	Algorithm	N	N	Y
Sprites improperly clipped		Algorithm	N	N	N
Stairs create unknown sector types	Undisputed	Algorithm	Y*	N	N
Stairs that can be ascended, but not descended	Disputed	Algorithm	N	Y	N
Starting on a vertex between visplanes causes illegal memory access	Undisputed		N	Y	N
Status bar face hysteresis	Disputed	Algorithm	N	N	N
Straferunning	Undisputed	Algorithm	N	N	Y
Tag 666 idiosyncrasies	Undisputed	Algorithm	N	Y	N
Tag 667 idiosyncrasies	Undisputed	Algorithm	N*	N	N
Targets not preserved in saved games	Undisputed	Algorithm	N	N	Y
Things block floors and ceilings when they should not	Undisputed	Algorithm	N	S	N
Things snap up to ledges when sectors move	Undisputed	Algorithm	N	Y	Y
Turning resolution is lowered when recording demos	Undisputed	Static limit	N	N	N
Tutti-frutti effect	Undisputed	Linedef	N	Y	N
Unused frames and obstacle sprite [4]			N	Y	N
Venetian blind crash	Undisputed	Multiple	Y	N	N
Vertical cutoff of partially invisible sprites	Undisputed	Static limit	N	N	N
Vertical offsets are ignored in texture patches	Undisputed	Algorithm	N	N	N
Visible sprites limit	Undisputed	Static limit	N	S	N
Visplane overflow	Undisputed	Static limit	Y	S	N
Voodoo doll	Undisputed	Algorithm	N*	Y	N
Wallrunning	Undisputed	Algorithm	N	N	Y
Weapon pickup message not displayed in multiplayer		Algorithm	N	N	N
Weapon sighting is off center			N	N	N
Wrong sound for exit switches			N	N	N
Wrong sound origin for button "popouts"	Undisputed	Algorithm	N	N	N
Wrong sound origin for switch/button presses	Undisputed	Algorithm	N	N	N
Z_Malloc error	Verified	Algorithm	Y		N
Zero press	Undisputed	Roundoff	N	S	Y

Bugs present in versions prior to Doom II version 1.9 for DOS

Phenomenon	Canonicity	Cause	Fatal?	Fixed in version	Workaround?	Loophole?
Armor percentage rollover	Undisputed	Static limit	N*	1.25?	S	N
Barrel suicide	Undisputed	Algorithm	N	1.5?	N	Y
Broadcast packet meltdown	Verified		N*	1.2		N
Crushed monsters block player movement	Verified	Algorithm	N		N	N
Health limited to 199		Algorithm	N	1.25?	N	N
IRQ 2 does not work	Verified			1.2
Items respawn with pickup flag off	Verified	Algorithm	N
Moiré error	Undisputed	Overflow	N*	1.4	S	N
Network game crashes with 3 or 4 players	Verified		Y			N
Network play locks up when using a sound card	Verified		Y	1.2		N
Projectiles triggering linedefs	Verified	Algorithm	N	1.4	N	Y
PS/2 mouse bug	Verified			1.666
Retriggering an open door after loading a savegame causes crash	Verified	Algorithm	Y	1.2	N	N
Savegame slots 5 and 6 corrupt other files	Verified		N	1.2		N
Skill level cannot be selected in some network games	Verified		N
Switches and lifts deactivated when restoring a saved game	Verified	Algorithm	N		N	N

See also

• Doom change log
• Doom source code oddities
• Static limits
• Engine bugs in Heretic
• Engine bugs in Hexen
• Engine bugs in Strife