The Visplane Overflow is a fatal error that occurs in Vanilla Doom when there are more than 128 unique floor and ceiling surfaces (visplanes) on the screen simultaneously. The visplanes that cause overflows are most usually the furthest away from the player. The restriction on the number of visible visplanes was eliminated completely in BOOM; removal of the visplane limit is a standard feature of almost all modern source ports. For technical information about visplanes, see Doom rendering engine.