The explosions of rockets and barrels only check the horizontal position of an object when determining how much damage to do to objects within their blast radius. As a result, the pattern of damage is a square box of infinite height rather than a sphere. The effect is most noticeable when firing a rocket against a wall high above a monster or player. The victim will be injured or killed because it is within the horizontal blast area, despite standing far below the point of impact.
An example of this phenomenon occurs in MAP30: Source Control of Hell Revealed II: Romero's head lies in a cylindrical enclosure far below the wall that is the intended rocket target. Nevertheless, the head suffers blast damage and therefore may be destroyed.
This situation has similar roots to solid objects making the space above and below them solid, and the ability of monster's melee attacks to hit at infinite vertical range.