Hi Andre, yes nonmanifold geometry can behave unpredictably.

Some mechanisms can do things like say determine orientation by intersecting a ray from a known outside point to a vertex and if it doesn't happen to cross the non manifold region it will seem to be ok.

Also another thing is that can be problematic for 3D printing in general are "thin" areas where 2 different points on the model surface are closer together than the building tolerance.

So you kind of want to avoid really pointy needle thin shaping like these:



- Michael