Usually non-manifold conditions, e.g. tangent contact or opposite corners touching along edge or point. Sometimes it's a surface irregularity that the parasolid kernel no likey.
Here is the problem I was trying combine the spiral bottom with top portion. THe top par was solidified based on surface I got from the bottom part. So I had alot of tangent contact
Tangent contacts are one of the "non-manifold conditions" that Roland mentioned. It's a limitation in SolidWorks that you just have to work around, either by leaving tiny spaces or tiny overlaps where the tangency should occur.
Hello Andi,
Can you post the part?