My response to this: http://modsonline.com/MapErrors-verr-77.html

I will notify Foyle that the map errors section is hard to find without a search and commenting on it seems impossible.

This error is explained very bizarrely and might well come from the RTCW days of mapping.

The issues here are how could detailing brushes help the max winding counts when terrain also effects this and yet terrain is always detail.

So the only explanation in my opinion is the "per cell" part which simply means "per portal".

So it's NOT the maximum limits of your map, but the maximum limitation of a portal, which if you have a skybox, that's 1 portal cell which means the entire map is being calculated at once.

So, making brushes detail allows the engine to 'draw' them only when the player can see the portal that said detail brush is contained within.

Solution:

Always start a custom level with a BSP Blockout. This is a basic layout which consists purely of large brushes (blocks) that represent where buildings/large walls will go. You can then portal your level. Once it's done and working everything you add should be detail (terrain and models are always 'detail'). In CODUO/2 you must compile VIS and then play. If you hit max windings per cell then you should get a brush number, where this brush is within the level will tell you exactly what portal is over the limit.

As the vertex count on a patch is mentioned in the FAQs section then I would assume everything that is calculated visually (so clips/nodes etc I am unsure of) counts towards the max winding. In UO I would expect it to be low but in comparison with stock levels at the time there's no reason why a custom map can't mirror the same detail.

Fixes would include firstly making all small brushes detail (that is making all small brushes in ALL OTHER portal cells detail as detail brushes in the corrupted cell won't make a difference on windings as they will always get drawn when the player is inside but any surrounding cells that see into this cell that are being calculated will be the culprits and the first ones to enhance the detail brush count), removing any unessential models (clutter objects for less visual loss) and most certainly decreasing the vertices on patches. A lot of people overcomplicate their patches, so only use what you need and the cut out the rest.

Hope that helps, it's a slightly more educated guess at what's going on here.

Okay, thanks guys!!!