Ok I've been having this problem for about a month now. At first I thought it was my software installation since i had reformated my machine and reinstalled. However I've tracked it to CADspan's STL surfacing in Sketchup. I'm pretty new to 3d modeling so I'm learning in google sketchup. In order to upload here I started using the demo version of CADspan. For the past month whenever I resurface a model to get a Watertight STL with CADspan parts of the model litterally disappear. I don't mean small holes appear but entire arms off of the figures i'm trying to build vanish. A freind who's a bit more experienced than me exported a model to 3ds max 2010 and then to STL. the STL file he made wasn't water tight but it did have all the parts. afterwards we tried processing it through CADspan's software again and it had missing parts again. It seems random what vanishes. I processed the same file multiple times and got different parts vanishing each time. I asked the CADspan support people and they didn't have an answer, they just told me the model error was caused by sketchup but gave no suggestion how to fix it or indeed what "error" was causing this. Has anyone else had this problem? and if so whats causing this and how can I correct it?