When damaged in melee by an attacker, a spiderdemon very occasionally becomes frozen in place, unable to walk or fire. Because monsters tend to repeat melee attacks until the target moves out of range, the spiderdemon then continues to take damage without retaliating. This stops only when the spiderdemon dies, or when a third party intervenes by assaulting one of the participants.
This error occurs because the spiderdemon's diameter is too large. When clipping checks are done in Doom, an allowance is made for an overlap into adjacent blockmap blocks by up to 64 units in any direction. Any monster which extends more than this may appear to be missing part of its bounding box when other monsters or players try to move against it. This allows those monsters to pass into the spider. However, when the spider itself subsequently attempts to move, it detects that it has a solid object inside it and thus its moves are rejected, causing it to stand in place.
This bug may also, but less often, affect smaller monsters such as the arachnotron.