I am having issues with some non-manifold geometry.. I understand there are a few types or a few sources of non-manifold geometry, but I can't find the exact issue with my model..
I noticed most of it, if not all, was located where I merged vertices together. Why would this cause non-manifold geometry? I thought the goal of the merging process was precisely to make sure I had a unified mesh?
Here is the file in question, maybe I missed something: