Engine bug

There are numerous bugs, limitations and oddities present in the Doom engine. For errors in map design, even those related to a specific item below, see the article about that map.

Verified engine bugs
These have been verified by id Software as bugs.
 * (none?)

Undisputed engine bugs
These bugs are obviously bugs (confirmed in the source code or by observation) but not yet verified by id Software.
 * (0,0) respawning bug
 * Barrel suicide
 * Chaingun fires twice with only one bullet in it
 * Fast doors make two closing sounds
 * Fast doors reopening with wrong sound
 * Ghost monsters
 * Ouch face
 * Sound effects behave differently on level 8 (Bug for Doom 2)
 * Sky never changes in Doom II

Advantageous bugs
These are obviously bugs, but possibly became regarded as features later.
 * Gamma correction resets palette
 * Rocket passes through the player who fired it
 * Silent BFG trick
 * Straferunning
 * Wallrunning

Engine limitation bugs
Certain engine limitations are often considered bugs.
 * Blast damage has unlimited vertical range
 * Hall of Mirrors effect
 * Map size limit (fatal)
 * Monster targets not preserved in saved games
 * Moving platforms limit (fatal)
 * Some game options not preserved in saved games
 * Lift heights not preserved in saved games
 * Revenant tracers not preserved in saved games
 * Turning resolution is lowered when recording demos
 * Visplane overflow (fatal)
 * Visible sprites limit

Sector bugs
These bugs are usually attributed to adjacent sectors, blockmap strangeness, or oddly built subsectors/nodes, or in the way the engine handles them. Many of these can be fixed covered up by editing the map in question.
 * Barrel explosions which do no damage at point-blank range
 * Hitscan attacks hit invisible barriers in large open areas
 * Flawed collision detection (aka hitscan attacks inaccuracy)
 * Long wall error
 * Mancubus fireball clipping
 * Monsters attacking across tall ledges
 * Player corpses walking
 * Player stuck on ledge after restoring savefile
 * Sprites flickering across ledges or lifts
 * Stairs that can be ascended, but not descended
 * Wrong sound origin for button "popouts"
 * Wrong sound origin for switch/button presses

AI limitation bugs
As the monster AI was quite advanced, of course it was buggy. Sometimes monsters do weird things.
 * Arch-Vile gluing monsters together
 * Lost Soul charging backwards
 * Lost Soul colliding with items
 * Lost Souls trapped outside level boundary
 * Lost Soul limit
 * Monsters fleeing
 * Monsters stuck in doortracks, walls or hanging off lifts
 * Sleeping Sergeant of Doom II MAP02
 * Spider paralysis

Bugs fixed before 1.9 releases
These might no longer be considered bugs as they were fixed at some point.
 * Projectiles triggering linedefs
 * Loading a saved game under an open door causes crash

Disputed bugs (or Map bugs)
These bugs have a high probability of being actual features or simply mapping errors. They could eventually be moved or removed once consensus is reached.
 * Commander Keen missing with -nomonsters
 * Dead player's line of sight tracks his killer
 * Invulnerability colormap bug
 * Monsters open locked doors

Unsorted

 * Demo desyncing caused by bouncing Lost Souls
 * Demo desyncing caused by menu access
 * Elastic collisions with walls
 * Frag count rollover
 * IDCLEV idiosyncrasies
 * Medusa effect
 * Moire error
 * Noclip bug
 * Non-Newtonian motion of corpses
 * Phantom fireball explosions
 * Picked up a medikit that you REALLY need!
 * Projectiles blocked by decorations
 * Savegame buffer overflow
 * Segfault/lightning flash near very tall structures
 * Sky hack
 * Slime trails
 * Spawn fog in wrong location
 * Status bar face hysteresis
 * Tag 666 idiosyncrasies
 * Tag 667 idiosyncrasies
 * Teleporters in Final Doom don't set the Z coordinate
 * Tutti-frutti effect
 * Venetian blind crash
 * Voodoo doll
 * Wrong sound for exit switches