For building an A7 BSP Map, the invisible intersecting parts of blocks are clipped off by the build process, and new edges and vertices are automatically placed at the intersections. This makes rendering faster, because the overlapping parts of blocks aren't rendered (in a Simple Map, all blocks in the view frustum are just rendered over each other). The disadvantage is that small angle differences can lead to large vertex position differences. If surfaces touch or intersect at a very small angle, inaccuracies of only 1/100 degree can move the joining edge position by several pixels, which is really visible in the level.

For this reason, don't use extremely narrow, elongated, or acute blocks for a BSP Map . Do not create blocks with edges smaller than 2 quants. Always try to design your map with as less surfaces as possible, and use as thick blocks as possible. Sometimes you can't avoid surfaces touching at small angles - but even then there's a bad way and a good way to do it. For instance, if you have a sloping road over a hill (side view):