when doing outer shell or union over two solids, the software could not combine solids correctly, leaving internal faces which causes problems afterwards. My guess is this is a bug. Attached a file of two solids right before outer shell or union operation.
Here is a video of what i tried: solid inspector, cleanup, intersect with model, etc.
Also attached the skp file right before outshell or union operationDemo of outershell bug 10-15 forum discussion.skp (354.1 KB)