Do you have any special models or prefabs? they may have been screwed up, or you somehow turned a brush inside out, no idea how that could have happened - but some entitys like platforms seem prone to this. You can also look for a brush with bad extents, or check the texture, is it divisible by 16 on the edges EXACTLY? Finally, are you using the most modern version of zoners tools? Old versions or ancient qtools are more prone to this error