Sometimes those problems are not so obvious. As mentioned, WED doesn't like very narrow, elongated, or acute blocks, like extremely long thin plates. They may not only produce visible 'holes' in the level, but also gaps in the collision detection hull. Also avoid small angle differences between blocks. If you get a warning message, you can ignore it; warnings normally are not harmful to the level. But if you see a visible fault in the level, try to find out what the problem is, fix it, and re-build. Mostly the problem is connected to a certain block number. Use the Find Block command to examine the block in question. It may happen that after deleting a suspicious block, another one will show up with the same warning. The build process issues only one warning of the same kind at a time, so if there are dozens of faulty blocks, only the first one will be announced.

Another type of error messages may be caused by a too huge or too complex level. Such errors are indicated as'Map size - too much blocks', 'too much surfaces', 'too much portals' oder 'visibility list overrun'. In that case the solution is painful, but simple: remove a couple of blocks, or a building. Otherwise there will be not enough memory for the level. If you really need a level such huge - like for building a whole city - you can try to split it into several map entities and build them separately. Then place them together into an almost-empty level that just contains a sky box, and a ground plate with sun light activated to avoid fullbrightness. This will reduce the frame rate a little, compared with building everything together, but sometimes it's the best solution.

Some errors are not indicated, but are visible in the level. The BSP tree process divides the map into regions along all non-detail surfaces. This can lead to heavy subdivision of surfaces in rooms with complex geometry. You can see the subdivision in the level by pressing the [F11] key two times while running the engine. Some 3D cards render 'T-junctions' - where 3 surfaces connect - inaccurate, which can lead to visible 'seams' in the level. For this reason, the BSP tree compiler tries to avoid T-junctions whenever possible. Another problem can arise when surface sections are smaller than one shadow map pixel (= 16 texture pixels). In some rare cases, a light 'shadow seam' can be visible along the edge of this surface. These problems are no 'bugs', but are inherent to the BSP tree process and the shadow map resolution, and are normally not visible. Otherwise they can be solved by simplifying the geometry, or using more detail blocks.