I've experienced what I think is an S3D "glitch" today and it's about made me pull my hair out trying to troubleshoot the problem. This part is designed as open frame, and I've used S3D to print it probably hundreds of times. Today when I went to print it, S3D wants to suddenly fill in the open frame area. I've run the part through just about every repair utility I have access to, including:
Meshmixer
netfabb
Microsoft 3D online repair service
Sketchup's Solid Inspector (Sketchup was used to create the part in question)
S3D's repair tools
All of the above find no problems with the part, and yet S3D insists on filling in the open area. It's never done this before, which tells me it's some sort of software glitch or perhaps I nudged some tiny S3D setting without realizing it and doing so caused the problem. I'm uploading several files to see if anyone else would like to take a stab at this.