Hmmm, there seems to be 3 types of bsp errors.
Ones which only appear when you look at them from a certain angle, or that don't appear when you are inside a wall. These seem to be caused by some sort of intersection/order problem. Mainly with non-solids and sheets.
The errors which are always visible, these seem to easier to fix and find the cause of the problem than with other errors. Tey seem to be caused by vertecies off the grid, areas with too many awkward bsp cuts etc.
Also seem to appear with some warp zone errors I think.
Collision errors, don't know what the hell causes these but they are USUALLY easily fixed by finding the path of the collision error then adding a brush somewhere in it's path.
Blah blah blah I have no idea what I am talking about blah blah blah