Hi Alexander, yes can you please post the 3DM file, that would make it easier to examine what you are running into.

But part of the process of Shell involves doing an offset of surfaces and if the surfaces are not smooth where they touched it will also try to extend the offset results.

So you may be seeing the result of that extension process.

But it's a bit hard to tell just from the screenshot without seeing the actual model file - if your surface is kind of lumpy or wrinkled or folded back on top of itself, that can cause a lot of problems for shelling as well because creating a surface offset tries to make a new surface that is moved away from the original along the original's surface normal, and if the original surface normal is going kind of crazy jiggling around too much instead of nice and smooth it can make it difficult to create a good offset. Basically any folds or jiggles in the original surface tend to get magnified when creating an offset from it.

One thing you can try in cases like this is to use Edit > Separate on your model and then use Offset to create offsets of the individual surfaces. If those look wonky then you may have some kind of bad quality surface that has normal/wiggling/folding problems in it. If the individual offsets look good then you're probably running into problems around the surface extension mechanism in shell - to fix stuff like that means making a better quality initial model that has pieces that are smooth where they touch each other instead of meeting in a crease.

But in general Shell is a sensitive operation and it does not tolerate messy situations very well... So if you intend to shell a model you usually have to make the original shape to be a nice clean starting shape.

- Michael