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.