Hi eric - it's a limitation of the geometry library that MoI uses. It can't deal with multi-surface offsets very well where the offset would result in a different type of vertex structure than the starting object. The particular problem area area is here where there are 4 surfaces meeting at single vertex, with one of them being a sharp spike coming to a point:



And actually when I examined a bit more closely I saw that they don't meet at a point down below, there is a small extra line segment there and also another problem is the at the top of that little slivery piece the surface seems to fold back over on top of itself:



Also your particular case here where you've got surfaces that are smooth one end where they touch but sharp on on the other end is probably a difficult case for the offsetter to handle - at smooth areas offset surfaces naturally touch each other and at sharp areas a gap opens up between them and so the surfaces need to be extended to fill in the gap. When they are extended the smooth areas will have overlapping surface area and be difficult to intersect with each other.

Probably to get a clean solid as an end result you would need to manually consruct the core piece and boolean it away rather than trying to use Shell for this particular object.

- Michael