Can sometimes happen when and entity cuts into a wall or alike and on rendering the engine sends the polys off into a tangent in all directions. Segments use entities and its often these which will cause a problem when itersecting semgents are formed from entities. In BSP rendering faces must be convex - concave faces will cause errors - I believe that is correct someone help me out here it may be the other way around - so bad faces are a common sources of this problem. Something like that anyway.
Moving an entity slighty so that its ploys do not intersect with anything else can fix it. Even just deleting the entity and replacing can fix it. A matter of trial and error. Isolate the offending entity and try and do something about it and it will go away.
I have had it happen numerous time for instance using a default metal staircase segment which cuts into walls either side - also happens with windows that cut through segment walls. In BSP rendering when anything cuts through another object the engine has to decide how to seal the cut section and reassigns the polys on compile - occasionally some types of faces can cause problems for BSP engines and the result is as seen in extreme circumstances.
The result can be it can seriously affect gameplay speeds too as quite obviously the serious nature of the result causes errors in the level and the math and the FPSC engine is not happy when any such errors occur.
Anyway - redo the offending part of the level and it should dissapear - if your lucky - try another different segment or entity.
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