Abnormality 0018, known as "The Walls Can Bleed," is a phenomenon tied to map textures in Garry's Mod. The anomaly manifests spontaneously on certain maps, primarily indoor environments with brush walls using default Half-Life 2 concrete or plaster materials.
When triggered, walls will begin to emit dark red liquid that behaves like Source engine decals but with properties inconsistent with the base engine.
The liquid:
Flows in a continuous animated flow, unlike decal blood.
Leaves persistent puddles that act as physics objects, slowing player movement when stepped in
Produces an audio distortion resembling muffled crying or heartbeat sounds
This effect spreads to multiple surfaces over time. Walls, ceilings, and even static props may exhibit the bleeding effect until the map appears alive with shifting stains.
Standing too close to affected walls often drains health slowly, with no visible attacker. NPCs enter a panic state, ignoring the player enemy or not to shoot at the walls and run away if possible.
Abnormality 0019 was first discovered in 2011 on a friend server playing on gm_construct. They reported "walls started crying blood when we spawned NPCs." At first believed to be a custom blood mod, it persisted after all addons were wiped.