Hi,
I have two problems.
1.Mesh extract
When I do a mesh extract on my mesh, it always etracts weird polygons at the seams of the previous local subdivisions. No matter where the masking is laid in, it will always extract the same weird polygons with it. I cleared the mask before, checked with Flat material, and also dragged mask, nothing helps. the attached image shows the polygons that are extracted with.(see screenshot)[
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both problems on the same mesh, i tried to unify, or export/import as .obj, nothing helped.
Things to try.
Try checking the mesh integrity with Tools > Geometry > Check Mesh Integrity. Any errors?
Try importing it as obj with the weld function set to a max of 0.01 with Tools > Import > Weld.
Also try doing an auto groups on the whole model with Tools > Polygroups > Auto Groups. Do any solitary faces show up under poly frame view as individual groups?
-Checking the mesh integrity didn’t reveal any errors
-importing and welding didn’t change anything
-autogroups creates one group for the whole mesh.
I finally solved this by having a close look at the mesh in blender. There were some overlapping/weird shaped polygons in the mesh.
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So when I subdivide 2 times(with smoothing), the problems are all gone, the mesh extract, as well as the polygroups error. This doesn’t make much sense, because the polygroups error occured in a totally different place were all the polygons are fine with correct normals, etc., but anyway, it works now.
thanks for your tips that pushed me in the direction of checking the mesh topology. (unified skin … do I have to say anymore? :D)
YAYYY! Good old Blender! 
Yeah, there are so many weird things can happen to a mesh. Fortunately this kind of weirdness only occurs rarely in Zbrush.
Sometimes just loading an OBJ in another 3D app and saving it out again can fix things too.
There are a few more free ones that you may already know of that I use from time to time. Off the top of my head there is ReMESH and MeshLab.
http://remesh.sourceforge.net/
There are others, but I can’t think of them right now.
Anyway I’m glad you found a way to get the bugs out! If you have anymore questions don’t hesitate to ask. 