When a Doom player fires a weapon, the center of the weapon sprite is actually a few pixels to the right of the line of fire (center of the screen, which is also the center of the status bar face). This discrepancy is not normally relevant, because it produces only a small targeting error at short to medium ranges, and because ordinary "bobbing" displaces the weapon sprite by a much larger amount. When aiming carefully at long range, however, the bug becomes apparent.
In some ports such as the Playstation version, this is very easily noticeable from firing the Rocket Launcher at anything on a different floor height.
Many source ports, such as ZDoom, solve this problem by including a crosshair in the in-game view. In ports whose automap includes an overlay mode, the player marker can also serve as a crude crosshair.