Flawed collision detection

Due to a coding oversight in vanilla Doom, a hitscan attack will pass through its target if it does not enter the block in which the center of the target thing is located. This is often called the blockmap glitch or blockmap bug.

Another issue is that a monster's cross-section, rather than its bounding box, is used to check for hitscan impacts; this makes melee attacks unlikely to hit very wide monsters like the mancubus, spiderdemon, and arachnotron.

Many source ports retain this flawed behavior. However, ZDoom mitigated it with a more robust method.

The bug can occur anywhere, even in open areas, as it is dependent on the alignment of the player and the monster(s) at which the player is firing. However, it is especially noted for being problematic in certain closed areas of particular maps such as narrow hallways which happen to span across a boundary between blocks.