This can even show up in Drafting.
If you create a section view and accidentally make your cutting line tangent to a hole.
Seems funny but I think it employs Parasolid to do this.
Make sure Select in Assy is set to "Face Priority" and select the faces... Edit away! Best thing about it is that you can select multiple faces across multiple synch parts and edit them all at the same time.
My understanding is that Parasolid can quite easily model non-manifold geometry but that it was an executive decision by the developers to not allow non-manifold geometry in Solid Edge because it is non-manufacturable.
Here is an example of thickness error ...
The lower line of the new sketch is coincidence with the upper one of the 3D model.
You can have zero thickness or non-manifold features if they are in separate bodies in a multi-body part.
That was the work around that I just used to be able to move on, creating a New Body. I'm just happy that this isn't a part that I am having to design for us to make. It is an injection moulded plastic part - having never worked with injection moulding before, I don't know if a multi-body part would work for that.
Still though, I would have spent FAR less time on this if there was something as simple as the line that was the problem being changed colors for easy location. Minutes instead of hours.
I have no issue with wanting more hints from the software. Surely it must know where the problems lie. Or maybe not IDK.
If you attempt to union the bodies it will show you the problem area. This is a little late though.