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.

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, 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.

• Fixed in 1.9 — Some bugs appear only in versions of Doom prior to v1.9.

• 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).

List

Phenomenon	Canonicity	Cause	Fatal?	Fixed in 1.9?	Workaround?	Loophole?
(0,0) respawning bug	Undisputed	Algorithm	N	N	N	Y
AASTINKY and AASHITTY	Undisputed	Algorithm	N	N	N	N
All-ghosts effect	Undisputed	Overflow	N*	N	N	N
Armor percentage rollover	Undisputed	Static limit	N*	Y	S	N
Barrel explosions which do no damage	Undisputed	Algorithm	N	N	N	N
Barrel suicide	Undisputed	Algorithm	N	Y	N	Y
Blast damage has unlimited vertical range	Undisputed	Algorithm	N	N	N	Y
Broadcast packet meltdown	Verified		N*	Y		N
Bullet puffs do not appear in outdoor areas		Algorithm	N	N	N
Chaingun makes two sounds firing single bullet	Undisputed	Algorithm	N	N	N	N
Crushed barrels leave behind a pool of blood	Undisputed	Algorithm	N	N	Y	N
Crushed monsters block player movement	Verified	Algorithm	N	Y	N	N
Demo desyncing caused by bouncing lost souls	Undisputed	Algorithm	N	N	N	N
Demo desyncing caused by menu access	Undisputed	Algorithm	N	N	N	N
Drawsegs overflow	Undisputed	Static limit	N	N	S	N
Elastic collisions with walls			N	N	N	Y
Excessive scrolling texture overflow		Overflow	Y	N	Y	N
Fast doors make two closing sounds	Undisputed	Algorithm	N	N	N	N
Fast doors reopening with wrong sound	Disputed	Algorithm	N	N	N	N
Flawed collision detection	Undisputed	Algorithm	N	N	N
Floors and ceilings moving silently	Undisputed		N	N	N	Y
Frag count rollover		Overflow	N	N	N	N
Gamma correction resets palette	Undisputed	Algorithm	N	N	N	Y
Ghost monsters	Undisputed	Algorithm	N	N	N	N
Hall of Mirrors effect (limit)	Undisputed	Algorithm	N	N	S	N
Hall of Mirrors effect (missing texture)	Undisputed	Linedef	N	N	Y	N
Health limited to 199		Algorithm	N	Y	N	N
Health percentage hysteresis in sector type 11		Algorithm	N	N	N	N
Hitscan attacks hit invisible barriers in large open areas		Algorithm	N
IDCLEV idiosyncrasies			N	N	N	N
IDMUS requests invalid music		Algorithm	Y	N	N	N
Invulnerability colormap bug	Disputed	Algorithm	N	N	N	N
IRQ 2 does not work	Verified			Y
Items respawn with pickup flag off	Verified	Algorithm	N	Y
joyb_speed magic numbers		Overflow	N		Y	Y
Kill percentage flag used for -nomonsters and respawning	Disputed	Algorithm	N*	N	N	Y
Lift heights not preserved in saved games	Undisputed	Algorithm	N	N	N	N
Linedef type 37			Y	N	Y	N
Linedefs can be activated regardless of player's Z position	Undisputed	Algorithm	N	N	N	Y
Long wall error	Undisputed	Roundoff	N	N	S	N
Lost soul charging backwards	Disputed	Algorithm	N	N	N	Y
Lost soul clipping problems	Undisputed	Algorithm	N	Y	N	N
Lost soul colliding with items	Undisputed	Algorithm	N	N	N	Y
Lost soul limit	Undisputed	Static limit	N	N	N	Y
Lost soul target amnesia	Disputed	Algorithm	N	N	N	N
Lump size limit		Static limit		N	S	N
Mancubus fireball clipping	Undisputed	Algorithm	N	N	N	N
Map size limit (blocks)	Undisputed	Static limit	Y	N	S	N
Map size limit (integer overflow)		Static limit	N	N	N	N
Medusa effect	Undisputed	Algorithm	N*	N	Y	N
Menu error with high mouse sensitivity	Undisputed	Overflow	Y	N	Y	N
Moiré error	Undisputed	Overflow	N*	Y	S	N
Monsters attacking across tall ledges	Disputed	Algorithm	N	N	N	N
Monsters fleeing	Disputed	Algorithm	N	N	N
Monsters open locked doors	Undisputed	Roundoff	N	N	N	Y
Monsters stuck in doortracks, walls or hanging off lifts	Undisputed	Algorithm	N	N	N	Y
Moving platforms limit	Undisputed	Linedef	Y	N	S	N
Network game crashes with 3 or 4 players	Verified		Y	Y		N
Network play locks up when using a sound card	Verified		Y	Y		N
Noclip bug	Undisputed	Overflow	N	N	N	Y
Non-Newtonian motion of corpses	Disputed	Algorithm	N	N	N	Y
Numeric overflow crash in tall areas	Undisputed	Overflow	Y*	N	S	N
Ouch face	Verified [1]	Typo	N	N	N	N
Phantom fireball explosions	Disputed	Algorithm	N	N		N
Picked up a medikit that you REALLY need!	Undisputed	Typo	N	N	N	N
Player corpses walking	Undisputed	Overflow	N*	N	Y	N
Player face grins after restoring savefile	Disputed	Algorithm	N	N	N	N
Player starts missing	Undisputed		Y	N	Y	N
Player stuck on ledge after restoring savefile	Undisputed	Algorithm	N*	N	N	N
Projectiles explode on impact with "sky"		Algorithm	N	N	N	N
Projectiles triggering linedefs	Verified	Algorithm	N	Y*	N	Y
PS/2 mouse bug	Verified			Y
Raising sectors change to orphaned sector type	Disputed		N	N	N	N
Ranged attacks blocked by decorations	Disputed	Algorithm	N	N	N	Y
Retriggering an open door after loading a savegame causes crash	Verified	Algorithm	Y	Y	N	N
Rocket passes through the player who fired it	Disputed	Algorithm	N	N	N	Y
Savegame buffer overflow	Undisputed	Static limit	Y	N	S	N
Savegame slots 5 and 6 corrupt other files	Verified		N	Y		N
Screen melt effect during Doom II cast sequence		Algorithm	N	N	N	N
Silent BFG trick	Undisputed	Static limit	N	N	N	Y
Skill level cannot be selected in some network games	Verified		N	Y
Sky hack	Undisputed	Algorithm	N	N	N	N
Sky never changes in Doom II	Undisputed	Algorithm	N	N	N	N
Sleeping Sergeant of Doom II MAP02	Undisputed	Typo	N	N	N
Slime trails	Undisputed	Roundoff	N	N	N	N
Some game options not preserved in saved games	Undisputed	Algorithm	N	N	N	N
Sound cutoffs	Disputed	Static limit	N	N	N	N
Sound effects behave differently on level 8	Disputed	Algorithm	N	N	N	N
Sounds changing pitch on slow computers			N		N
Spawn fog in wrong location	Undisputed	Typo	N	N	N	N
Spawn cubes miss east & west targets	Undisputed	Algorithm	N	N	Y	N
Spawn spots not preserved in saved games	Undisputed	Algorithm	Y*	N	N	Y
Spechits overflow	Undisputed	Static limit	Y	N	S	N
Spider paralysis	Undisputed	Algorithm	N	N	N	Y
Sprites flickering across ledges or lifts	Disputed	Algorithm	N	N	N	N
Stairs create unknown sector types	Undisputed	Algorithm	Y*	N	N	N
Stairs that can be ascended, but not descended	Disputed	Algorithm	N	N	Y	N
Status bar face hysteresis	Disputed	Algorithm	N	N	N	N
Straferunning	Undisputed	Algorithm	N	N	N	Y
Switches and lifts deactivated when restoring a saved game	Verified	Algorithm	N	Y	N	N
Tag 666 idiosyncrasies	Undisputed	Algorithm	N	N	Y	N
Tag 667 idiosyncrasies	Undisputed	Algorithm	N*	N	N	N
Targets not preserved in saved games	Undisputed	Algorithm	N	N	N	Y
Turning resolution is lowered when recording demos	Undisputed	Static limit	N	N	N	N
Tutti-frutti effect	Undisputed	Linedef	N	N	Y	N
Venetian blind crash	Undisputed	Multiple	Y	N	N	N
Vertical cutoff of partially invisible sprites	Undisputed	Static limit	N	N	N	N
Visible sprites limit	Undisputed	Static limit	N	N	S	N
Visplane overflow	Undisputed	Static limit	Y	N	S	N
Voodoo doll	Undisputed	Algorithm	N*	N	Y	N
Wallrunning	Undisputed	Algorithm	N	N	N	Y
Weapon sighting is off center			N	N	N	N
Wrong sound for exit switches			N	N	N	N
Wrong sound origin for button "popouts"	Undisputed	Algorithm	N	N	N	N
Wrong sound origin for switch/button presses	Undisputed	Algorithm	N	N	N	N
Z_Malloc error	Verified			Y

See also

• Doom change log
• Doom source code oddities