Wherever you have a sharp edge in blender, there must also be a UV seam (but you don't need a sharp edge on every seam). Also, don't use the edge split modifier on the exported mesh (delete it before exporting). Just make sure to check "Smooth Groups" when you export as .obj (this will convert your sharp marked edges to…
I mean that I split only edges (on the model) that are marked as sharp. And I don't know what do you mean in your second sentence :D Here it is how I'm doing that: - flatten mapping my UV and then stitching pieces - select hard edges on the model and mark them as sharp - apply Edge Split modifier with checked "sharp edges"…
FYI, blender can do per-face normals at a vertex without an edge split modifier now (as of 2.71). Set the object's shading to smooth, add sharp edges, and check "Auto Smooth" in the Mesh Data tab. All the pre-2.71 tutorials are going to tell you to use edge split for sharp edges, but it's no longer the only way.
Yep, it's a projection error. Using a cage solved it for me. Perhaps you know this but to make a cage in blender, just duplicate your lowpoly, then in edit mode press alt+s to push the mesh outwards until everything's covered, then remove sharp edges (not sure if this is necessary?) and export.