Z-fighting

Z-fighting is when two surfaces (brushes, props, any visible object) happen to share a geometry plane with the exact same angle and position. This causes the renderer to become confused and attempt to draw both faces at once. This results in the geometry looking very odd and unpredictably shifting when the player changes their view angle.

Z-fighting is commonly seen on overlays and decals inside Hammer, but this is only because Hammer doesn't render them how the engine does. The common solution to Z-fighting is to move or resize one of the objects just slightly.