Visplane
The Doom engine's container for a visible patch of floor or ceiling — and the error message that taught a generation of level designers restraint.
Doom (1993) draws walls first, column by column, in BSP tree order. Whatever is left above and below each wall slice gets collected into visplanes: one structure per unique combination of height, flat texture and light level, recording which screen columns it covers. Once the walls are done, each visplane is drawn as horizontal spans, since a flat floor has constant depth along a screen row.
The original code used a fixed table of 128 visplanes. Put too many distinct floor heights or light levels in one view and the game died with R_FindPlane: no more visplanes. Mappers learned to count surfaces in their heads, and tools that showed the number live became standard kit. Source ports after the 1997 code release made the table dynamic and the problem vanished — except for purists who insist on the original executable.