Object does not show the same form after slicing

I also had similar issues using sketchup files from 3dwarehouse exported to stl. due to some having hidden objects from designer errors etc. netfab will show them, then open the original sketchup file, select 'unhide objects' delete anything unwanted, save then export new file to stl and look at it in netfab again. thats how I fixed about 6 files. some are very hard to see and delete being near invisible. seems people think 'hide' is same as delete when designing. others use this trick to prevent their designs being copied even embedding name text etc hidden into inside areas of models. even found a photo on an inside wall of a structure (lots of that gets ignored during export to stl though same for colors thats why you see thousands ignored faces etc).