This never happened to me...strange. Are you using hmp or mdl terrain? Is the "to_floor_norm" flag set or not (in the behaviour-panel)? Make sure your building is placed above the terrain!

Does anyone else experiance this?

If you have a ocean/lake watersurface, make sure the river's z-position is slightly higher than the other water planes. Also make sure you don't have any camera.clip_far/clip_near set in your own scripts...

If you want, you can pm me a download link to your project and I will examine.