I was trying to model my object for 3d-printing and faced this problem. I use
3D Printing Toolbox in
Blender v2.79. And checking my object for manifoldness, it shows me that I have
Bad Contiguous Edges. I don't know what to do with it.
On this picture, that edges are selected. And 28 Non Manifold Edges are because I haven't used Mirroring yet.