Engine bug

There are numerous bugs, limitations and oddities present in the Doom engine. Errors in map design should generally go on the map's description page.

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
 * 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
 * Fast doors reopening with wrong sound
 * 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